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I am not an expert in GMC 4wd systems, but your theory is logical. Try to remove the CV joint first at the transfer case and see if you still have rear wheel drive. This will be easier than opening the front axle case and removing the components. Just take out the driveshaft to the front wheels differential and you should still have rear wheel drive.
Hi there: Seen a lot of the actuators on these vehicles put the transfer case in the wrong position and cause issues like this. You need a scan tool to watch the data on it and verify correct gear position.
To check the front differiental to see if it is locked in, you can remove the front actuator and see if it extends out when you place it into gear. or you can put it in 4wd, and drive it a few feet, then jack up the front tires off the ground and turn one tire and see if the other tire turns in the opposite direction, If so, the front diff is locked in and the issue is else where.
The place to start with this is to lift the front end of the vehicle. Turn the ignition on and place the truck in 4wd. Then when the front is safely supported, climb under and see if the front driver shaft will turn or if it is stuck. If it turns, then you have issues in the transfer case. If the front drive does not turn, then you need to remove the actuator on the front right and see if it extends in 4wd and retracts in 2wd. If not then you have an actuator or wiring issue. If it does then you have a problem with the sliding gear used to engage the front axle (caused for Damage, lack of lube, weak actuator). Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.
The first thing that I would do is get up under the vehicle and check the lube in the front and rear differential and the transfer case. The 4 wd may still engage but if it is low on lube it will go out soon.
Does the truck have a lift kit on it?
- CV Axle spacers may be needed.
Have you recently replaced the CV Shafts?
- You may have gotten an axle that was too short / too long if you got a rebuilt axle.
It might be a worn pinion bearing in the front differential. Those can get louder with more speed. Check for excessive play in the yoke.
I would also check the hub bearings on each front wheel. Its more of a growling sound than a whining sound. I have done both of mine in the last 3 years on my 2000 GMC Sierra. They run about $60 at Napa with a 1 year warranty. You will need a slide hammer.
Jack the truck up so the wheel is off the ground and shake it. If you can move it around, its bad.
There should be a 4wd fuse inside the driver door at the dash by front pillar. Remove cover look inside cover and find 4wd fuse, replace. If it blows again it will most likely need a front axle electronic actuator.
Check the actuator cable that activates the front axle disconnect. It is
bolted to the inner fender and the cable runs down to the axle. If you
apply vacuum it should pull in and shift the interlock. To see if it is
working, jack up the right front so the tire clears. The tire should
rotate pretty free. Now engage the interlock. The tire should become
harder to turn and the front drive shaft should turn. Now engage the 4X4
(with the trans in park) and see if you can still turn the tire. You
shouldn't be able to turn it with the interlock activated.
There should be a plug on the axle housing near where the front drive shaft meets front axle I think on drivers side inline with drive shaft.Remove plug,Fluid should be at bottom of hole.
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