I just bought an 05 Chevy uplander AWD, and every now and then my service abs system warning comes on skins with the all wheel drive disabled message... What could be causing this?

You have an all wheel disabled message because there is an error in the system.

i would start off by checking the abs cables and sensors, if one is broken or damaged it will keep the all wheel drive from working and the abs. if the technician let somethings hang or crimp the cable then it wont get a signal and shut down the abs and stabilitrak/awd if the cables are ok and if your able to get the abs sensor free from the hub check the ends of it, its essentially a magnet and they will get metal pieces stuck to them, also if a hub was messed with a bad hub can grind down the ends of the abs sensors. put antiseeze before you put the sensor back in because they can be hard to remove and can get broken off inside, which can be fun to try to remove. if you remove the abs sensors from the hub, if able to there should be a torx or allen, remove that then wiggle it gently out so it doesnt break. start with the cheapest and easist and work on. also a code will go a long way in diagnostics, a good scanner will tell you the speed from each wheel (live data) and that will tell you something if one wheel says your going 15mph and the other says 0, while the vehicle is going 15 mph then the one claiming 0 is bad. after checking the abs system move onto other things such as maybe stabilitrak, these problems can be fun to work on so dont get discouraged,
good luck!
also its recommended to change the hub(s) and not just the abs sensor if you find a bad, broken or damaged sensor.
and if i think of something else i will keep you posted.

Check switch that enable/disables it. If electronically controlled, it may use ABS, Traction control, power steering and gear mode data as it's input to enable/disable AWD. Check for error codes for these systems too. If all is OK, check operation of mechanical part such as AWD transfer box or the whole transmission for wear and tear. Some do have mode switch that driver can select between snow and normal or mud. If present check such item too.

AWD Disable indicator
The AWD Disable indicator is located in the instrument panel cluster. This lamp is used to inform the driver that the AWD system has been disabled and no torque will be applied to the rear wheels during a slip condition. The AWD Disable indicator is controlled by the powertrain control module via a class 2 message.
Powertrain Control Module
The powertrain control module monitors the data from the ABS controller and Rear Drive Module (RDM) for proper operating conditions. If inappropriate conditions are present the PCM commands the differential clutch pump actuator check valve closed, disabling the AWD system. The PCM also commands the AWD Disable indicator on.
Is the All-Wheel Drive Disabled Indicator Always On ?

Is

Hi! Matt, Most likely if the anti-lock brake light and the awd disable light come on at the same time a wheel bearing / wheel speed sensor assembly is faulty.Listen for a humming noise when driving and try to determine where it is coming from(left/right/front/ back) or any looseness/play in any wheel. That's the possible bearing/wheel speed sensor causing the problem. Or have the abs system scanned to determine which wheel sensor is reading incorrectly.If the awd disable light is off then the awd system should be funtioning.

Page 13 of owner's manual says:
AdvanceTrac
(if equipped):
Illuminates when the AdvanceTrac
system is active. If the light remains
on, have the system serviced
immediately.
AWD locked (if equipped):
AWD
Illuminates when all wheel drive
LOCKED
(AWD) is engaged/activated. If the
light continues to flash, have the
system serviced.

Page 224:
ALL WHEEL DRIVE (AWD) SYSTEM (IF EQUIPPED)
Your vehicle may be equipped with an All Wheel Drive (AWD) transfer
case. With the AWD option, power is supplied to all four wheels
automatically with no need to shift between two-wheel drive and
four-wheel drive. If your vehicle is equipped with the AdvanceTrac
stability enhancement feature, you can change AWD modes, if desired,
by selecting AWD options through the message center.
For the lubricant specification and refill capacity of the AWD transfer
case refer to Maintenance and specifications chapter.
If your vehicle is equipped with AWD, a spare tire of a different
diameter than the road tires should never be used. Such a tire
could make the vehicle difficult to control as well as result in damage
to driveline components.
Utility and four-wheel drive vehicles are not designed for
cornering at speeds as high as passenger cars any more than
low-slung sports cars are designed to perform satisfactorily under
off-road conditions. Avoid sharp turns or abrupt maneuvers in these
vehicles.
The following sections, Normal operation, Autolock operation and
Disabling AWD, apply only to vehicles which allow the driver to change
AWD modes through the message center.
Normal operation (vehicles equipped with AdvanceTrac
During normal operation:
• the AWD system is in AWD AUTO mode (AWD LOCKED will
illuminate in the message center for four seconds when you first start
your vehicle). The AWD LOCKED indicator in the instrument cluster
will also come on.
• AWD AUTO mode can be overridden by pressing SETUP in the
message center. When you first press SETUP, AWD <AUTO> will be
displayed, then if you press RESET within four seconds of pressing
SETUP, AWD <LOCKED> will be displayed. AWD <LOCKED> provides
power to all four wheels constantly which allows you to operate your
vehicle in severe winter or off-road conditions such as deep snow, ice
or shallow sand. It is not recommended that you use AWD <LOCKED>
on dry pavement.

Page 225:
• AWD <LOCKED> can be cancelled by pressing RESET in the message
center. (The AWD LOCKED display in the message center will then
turn off and the display will return to AWD <AUTO> for four seconds
when the shift is complete.)
Autolock operation (vehicles equipped with AdvanceTrac
If the AWD system begins to overheat, the system will place itself in the
Autolock mode:
• the AWD LOCKED indicator light will illuminate in the instrument
cluster and AWD DISABLED <LOCKED> will be displayed in the
message center.
• AWD LOCKED will be displayed in the message center for four
seconds and a warning chime will sound. This condition may clear
without any action being taken by the driver.
• the AWD LOCKED indicator light in the instrument cluster will turn
off when the AWD system cools down.
• When the system is sufficiently cooled down, AWD AUTO RESTORED
will appear in the message center for four seconds and a warning
chime will sound.
Disabling AWD (vehicles equipped with AdvanceTrac
When a problem is detected in the AWD system:
• the AWD LOCKED indicator light in the instrument cluster will flash
eight times every two minutes in the message center.
• The message center will then display AWD DISABLED for four
seconds, then SEE OWNER'S MANUAL. A warning chime will also
sound. The warning displays and chime will continue until RESET is
pressed (which will clear the message from the display).
• AWD LOCKED can still be engaged by pressing SETUP (the message
center will display AWD <DISABLED> LOCKED), then pressing
RESET. The AWD LOCKED indicator light will illuminate in the
instrument cluster and the message center will display AWD
DISABLED <LOCKED>.
• To disable AWD, press RESET. The system will go back to AWD AUTO
mode momentarily, then the AWD LOCKED indicator light will flash in
the instrument cluster eight times every two minutes. The display will
then show AWD <DISABLED> LOCKED for four seconds and then
turn off.

Page 226:
• If a serious problem occurs with the AWD system, the AWD LOCKED
indicator light in the instrument cluster will flash 10 times every two
minutes. The message center will also display CHECK AWD and a
warning chime will sound. You can press SETUP to check the status of
the AWD system (which will display AWD <DISABLED> LOCKED),
but the AWD system will be disabled until the system is serviced.
Driving off-road with truck and utility vehicles
AWD vehicles are specially equipped for driving on sand, snow, mud and
rough terrain and have operating characteristics that are somewhat
different from conventional vehicles, both on and off the road.
When driving at slow speeds off-road under high outside temperatures,
use 1 (First) gear when possible. 1 (First) gear operation will maximize
the engine and transmission cooling capability.
Under severe operating conditions, the A/C may cycle on and off to
protect overheating of the engine.

Sounds like you need new brake pads , when the pads get worn it triggers the warning replace the brake pads reset the computer this should fix the noise and the warning lights.

i had the same problem with my 2005 chevy uplander. the ABS lights kept coming on. The fix for me was... I had a damaged and old battery(the original battery). If your still under warranty or not I would suggest you check this out by using a new battery to see if this is your solution.

Your vehicle is equipped with vehicle stability control, what the system does is prevent the car from going out of control due to a skid, it does this by applying the brakes at each wheel very quickly to straighten the car out in the event of a skid like on ice or a wet road, the system is part of the ABS (Anti-lock brake system), when the light comes on and stays on it means that something has failed in the system and the computer that controls it has disabled the system

I would take it back to place where it was repaired, it is possible that the part they replace is defective..

Also they should have clear all codes in the system at that time!!!