Hey guys and probably girls as well. New guy here. Never been to this forum before but have a truck problem that I'm sure one of you wizards will know something about.
I have a 99 GMC extended cab, 5.3 Litre engine i believe. My problem is this. If I am stopped in a slippery situation, be it snow, ice or mud. I tend to be stuck there, meaning if one of my tires, usually a rear one starts to spin, my 4 wheel drive does nothing. The 4x4 is engaged before I stopped so i got to the location but now can't get out. Is there some sort of rear end differential lock that is not kicking if there is one at all. Or is this truck to old and maybe something else has failed. Thanks in advance.

Later
E

First, do you have a manual transfer case or push button?

2nd, look at your RPO sticker(usually in the glove box) and find a 3letter/number code: G80.

I'm wondering if your front differential is engaging. Best way to find out is to go find a dry parking lot big enough for you to do circles(slowly). Now, put it in 4HI, drive straight for a few feet, now turn the steering wheel either left or right to its full stop and continue moving. Depending on the vehicle, you may experience wheel hop(bark bark) or you may feel the vehicle try to stop which requires you to mash on the gas more. If none of the above happens while turning, your front end is not engaging. Let us know what happens.

Second with Mike, need to tell us how your truck engages 4WD, via push button or lever. If push button, 4WD engages via a vacuum associated with a actuator, among other things. It is common to have a vacuum leak prevent 4wd from kicking in on he push button Chevys. One quick way to check the 4wd vacuum is to check your interior vents. If air is always blowing through the same vent, even if you switch from dash vent to floor vent, you have a vacuum leak also prevents 4wd from kicking in.

Mike, my *-10 will contradict that statement. I am unable to change the vents via the control, (verified via the main vac line to the HVAC unit) but my 4X4 still works.

Anyhoo, back on topic.

everything on the full size is electronic.

Wait, the front diff actuator or the HVAC selector?

both, everything hvac and 4wd. i dont have a manual tc so those might have a physical cable to the tc not sure.

but you dont see to many of those so im guessing he has the autotrac with the buttons on the dash

Duly noted, thanks for the 4x4 course.

if i had to guess i would have the s10 size would have stopped using vacuum stuff a long time ago since they started with electronic actuators in early 90'* maybe even the 88 body change for c/k. i had vacuum actuators for everything on my old jeep and they were miserable in the cold and if you jam on the gas they would lock and unlock rapidly. i had a rear driveshaft go so i took it off and put it in 4wd to go home and i couldnt go over like 1/4 throttle