Have a 2002 4 wheel drive gmc.when I used to get up.to 55.now truck would only go into gear when its cold.but still birnx out when its cold mechani. Shop said transmission needs to be rebuilt.

Transmission failure, computer failure, electrical system issue. There are many possibilities

Vehicles: 2002 Chevrolet Trailblazer 4WD 4.2L SFI DOHC 6-cylinder

Your front 4WD Transfer case is low on Gear Oil.
On warm days, the Gear Oil expands slightly, and is less viscous, so the "Service 4WD" light stays off. On colder days/mornings the story changes.

The following paragraph has info applying to the manual differential only:
The 2002 Trailblazer (the first year Chevy made the Trailblazer) has a Front Differential using 1.11 (that's 1 and 1/9th) Quarts Gear Oil, and a Rear Differential with a capacity of 2.29 (thats 2 and 2/7ths) Quarts Gear Oil.
The Gear Oil needed is GM Part No.12345349 or GM Part No.1052931 (they're identical), also known as "GM Synchromesh"(R), which is a type of GLS (Gear Lubricant, Special), also known as "Manual Synchromesh Transmission Fluid 5W30". A synthetic version of which sells for $12.70/qt (USD) at
http://www.amsoil.com/storefront/mtf.aspx

This is not to be confused with 5W30 motor oil (which can be had for $3.00/qt (USD)).

The 2002 Trailblazer 4WD Transfer Case (NV126 Transfer Case) capacity is 0.8 Liters of GM Part No.12378508 (US), also known as "GM Auto Trak II"(R).
Note that one Quart (US) = 0.946 Liter, so 0.8 Liter = 27.06 ounces.

Red Line sells something that they say replaces both GM Synchromesh and GM Auto-Trak II for $12.14/Qt and $44.55/Gal (US) at
http://www.shopwiki.com/Red-Line-MTL-70W80-GL~4-Gear-Oil-%281-Quart%29

13 Manufacturer Recalls apply to the 2002 Chevrolet Trailblazer (model year 2002 was the first year Chevy offered the Trailblazer), and two recalls pertain to the 4WD Transfer Case. Note that one of the recalls applies to only 895 units, so your vehicle is unlikely to be affected by this Transfer Case recall.

2002 Trailblazer Recall ==========================
OCT 17, 2001 ' Recall ID# 54221

Recall Reason POWER TRAIN:TRANSFER CASE (4-WHEEL DRIVE)
Recall Date OCT 17, 2001
Model Affected TRAILBLAZER
Potential Units Affected 78004

Recall Summary
VEHICLE DESCRIPTION: SPORT UTILITY VEHICLES EQUIPPED WITH 4-WHEEL DRIVE. THE CALIBRATION OF THE TRANSFER CASE CONTROL MODULE COULD CAUSE INSUFFICIENT HIGH SPEED GEAR ENGAGEMENT.

Consequence
IF THE GEAR IS NOT ENGAGED, THE VEHICLE CAN ROLL WHEN THE TRANSMISSION IS IN "PARK" AND A CRASH COULD RESULT WITHOUT WARNING.

Remedy
DEALERS WILL UPDATE THE TRANSFER CASE CONTROL MODULE CALIBRATION TO INCREASE GEAR ENGAGEMENT. OWNER NOTIFICATION BEGAN NOVEMBER 6, 2001. OWNERS WHO TAKE THEIR VEHICLES TO AN AUTHORIZED DEALER ON AN AGREED UPON SERVICE DATE AND DO NOT RECEIVE THE FREE REMEDY WITHIN A REASONABLE TIME SHOULD CONTACT CHEVROLET AT 1-800-222-1020 OR GMC AT 1-800-462-8782.

Notes
GENERAL MOTORS CORP. 01064/01065

2002 Trailblazer Recall ============================
SEP 05, 2001 ' Recall ID# 26006

Recall Reason POWER TRAIN:TRANSFER CASE (4-WHEEL DRIVE)
Recall Date SEP 05, 2001
Model Affected TRAILBLAZER
Potential Units Affected 895

Recall Summary
VEHICLE DESCRIPTION: SPORT UTILITY VEHICLES (4 WHEEL-DRIVE). SOME OF THESE VEHICLES WERE BUILT WITH A RANGE SHIFT COLLAR IN THE TRANSMISSION TRANSFER CASE THAT CONTAINS CRACKS.

Consequence
THIS CONDITION COULD RESULT IN A COMPLETE FRACTURE OF THE RANGE SHIFT COLLAR AND SUBSEQUENT LOSS OF VEHICLE FORWARD AND REVERSE GEARS, AND THE INABILITY TO MAINTAIN "PARK" POSITION. THIS INCREASES THE RISK OF A VEHICLE CRASH RESULTING FROM UNINTENDED MOVEMENT OF A PARKED VEHICLE.

Remedy
DEALERS WILL REPLACE THE TRANSFER CASES AND UPDATE THE CONTROL MODULE CALIBRATION. OWNER NOTIFICATION BEGAN SEPTEMBER 6, 2001. OWNERS WHO TAKE THEIR VEHICLES TO AN AUTHORIZED DEALER ON AN AGREED UPON SERVICE DATE AND DO NOT RECEIVE THE FREE REMEDY WITHIN A REASONABLE TIME SHOULD CONTACT CHEVROLET AT 1-800-222-1020 OR GMC AT 1-800-462-8782.

Notes
GENERAL MOTORS CORP. 01061


Read about all (13) 2002 Chevy Trailblazer recalls at:
http://www.internetautoguide.com/10-12-2002-16-1287/2002-chevrolet-trailblazer-recalls.html

it sounds like ur rack in pinion is getting 2 go out

It's pretty basic actually. The encoder motor engages 4WD within the transfer case which drives the front differential. The drivers side wheel is always engaged at this point and the passengers side wheel is engaged via a vacuum controlled cable (actuator - lunder the battery). There are no "hubs" (so-to-speak), not in the way that you use to see on the older systems where you'd have to lock them.

its the 4 wheel drive module, if you do not switch into 4wd in the summer months to oil the gears, the computer will forget its a 4wd...dumb I know, sometimes they can reprogram at the dealership if not say good bye to at least 500.00

sound like shift motor is slipping trying to rotate sector shaft to go in gear.

differential planet gears worn the thrust washers away,To confirm this drain the diff oil and check for any metal particles.Solution replace the diff.Good luck.

bad leaf springs

sounds the wheel senser reading something not working properly

This sounds like ring and pinion noise, new bearings and a gear set will cure the problem.