Won't shift into 4 wheel drive mode at all / 2000 GMC Jimmy SLT

There are a few potential issues that could be causing your 2004 GMC Envoy SLT to go into 4-low but not 4-high or 2-wheel drive. Some possible causes include:

1. Transfer case issues: The transfer case is responsible for controlling the engagement of the different drive modes. If the transfer case is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
2. Shift motor issues: The shift motor is responsible for controlling the engagement of the transfer case. If the shift motor is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
3. Vacuum issues: The vacuum system is responsible for controlling the engagement of the transfer case. If there is a problem with the vacuum system, it may not be able to engage 4-high or 2-wheel drive.
4. Wiring issues: The wiring that controls the transfer case and shift motor can become damaged or corroded over time. This can cause the transfer case to not engage 4-high or 2-wheel drive.
5. Control module issues: The control module that controls the transfer case and shift motor can fail. This can cause the transfer case to not engage 4-high or 2-wheel drive.

It's best to diagnose the issue by a professional mechanic, who can perform a proper diagnosis and repair the issue. They will be able to determine the specific cause of the problem and recommend the appropriate repairs.

The reason the light is flashing constantly is because the vehicle is sensing that four wheel drive(4WD) is not working. These 4WD systems use vacuum from the engine to engage/disengage to front axle. The vehicle probably has no vacuum going to the front axle, which would not allow the 4WD shift to complete, which would then leave the light on the dash flashing.
First thing to check is that the vacuum actuator on the firewall is working as it should.

You mean drums? Depends on the vehicle options but should be rotors/pads. Easy to check - look through the wheel spokes and if pads, you should see the rotor.

may be bad starter

Fuel pump relay or fuel pump,or ignition switch

shift modual and check the transmission fuse

bad transmission, it will need rebuilding or replacement. sorry....hopefully this helps

Usually when a truck gets stuck in 4 low you need to put it in reverse, start backing up very slowly, put it in neutral (or push in the clutch on manual trans models) and stop the truck with the brakes. this will release the load on the gearing which should allow you to shift out of 4 low.

there is a transfer case mod. on the right panel were your fot would be on pass front seat . thats the problem . 2 open glove box and look up 3 digit code i think its np1 or np8 . then go to
car-part.com and buy used to save big very easy to replace remove plastic panel and there should be alum. box with 2 plugs hope this will get you going

do the lights work on selector buttons if so check actuator on transfer case may be bad or replace selector switch i just did and my four wheel drive works sometimes