2000 GMC Sierra Transfer Case or Transmission?

Son’s 2000 GMC Sierra, 200000 miles, 4.8L engine, automatic transmission and 4WD. Was driving home from a trip to the store - heard a “violent popping noise” on the passenger side and the truck simply slowed to a stop. Engine running fine, all sounds fine while idling. When we put it into any gear, I can “feel” it go into gear but the truck doesn’t move forward or back. Just changed differential, transfer case, and transmission fluids about two months ago.

I can offer any other information that might be helpful. Thoughts are appreciated!


What he might of heard was the sun gear reaction shell coming apart.

Enter GM Sun Gear Reaction Shell into a search engine and learn all about it.


Thanks @Tester, that gives me one more thing to keep an eye on as mileage grows on my truck. I hope this helps the OP, too.

@cliffdodge, sounds like your son needs a rebuilt tranny. While they are in there, make sure one of these replace the factory reaction shell.

Thanks for the help. I’ll post back when we know exactly what’s going on!

Did you use gm differential fluid, if not did you buy the recommended additive?

This would not be a stripped sun shell. With a stripped sun shell you will have First but no 2nd, Overdrive, or Reverse. If you can rev the engine high enough, the transmission might shift through 2nd to Direct but that is as far as it will get.

Barkydog, we used the oil that autozone recommended. I don’t recall exactly what it was, being new to 4WD trucks. No additive. Should I empty and refill making absolutely sure of the right fluids?

New development: changed the fluid and let the truck warm up. Shifted into reverse and it didn’t move but the speedometer moved with the speed of the motor. Put it into park and heard grinding. Thought to try it in 4WD. In 4HI, the truck drove in reverse then in forward.

Seems the front wheels are responding in 4WD, but the rear wheels are not engaging at all.


I had checked at autozone for rear dif fluid, and they suggested 2 tubes of anti slip stuff at 12 dollars a tube so went to gm and payed 30 bucks a quart for differential fluid. I had read of people not using and no trouble but if you have a limited slip rear end it might be advisable. This would really need a hands on diagnosis, can you see the drive shaft spinning when in gear in 2wd, with the proper safety precautions of course.

On standard differential rear axle assemblies, use SAE 80W-90 GL-5 (SAE 80W GL-5 in Canada) gear lubricant. On limited slip differential rear axle assemblies, use 4 fl. oz. of limited slip differential additive lubricant and SAE 80W-90 GL-5 gear lubricant."

Where was the grinding coming from?

The speedo is probably measuring the transmission output shaft rotation. So if that is registering speed, but the truck isn’t moving, it would imply the transmission is working ok, but the transfer case isn’t. On my Ford 4x4 truck , it has a separate lever for shifting the transfer case. 2 WD/4 High/4 Lo. If I get it in-between sometimes I’ll hear a popping or clicking noise. Maybe that is what is happening, you aren’t getting the transfer case fully engaged. By this I don’t mean there’s something wrong about the way you are operating the lever, just that something is askew somewhere between the lever and where it actuates in the transfer case.

An afterthought … Does this truck have locking front hubs? I’ve gotten popping noises from them if I forgot to set them correctly. Unlikely in your case b/c it appears the problem is in the rear drive, not the front.

Barky, not exactly sure where the grind is coming from, but will try to isolate. I like the idea of watching the drive shaft. Will do that as well.

George, the truck has an automatic 4WD setup, meaning we push the button and it goes. No lever, nothing manual. I’m more than leaning toward the transfer case now based on what I’m seeing and reading.

Thanks everyone! My first resource is always the best one - cartalk users!

Before I get into removal, is there anything obvious that I’m overlooking? Could a wire or other connection be disconnected? Anything that I should look for that could be causing a signal not reaching the transfer case? I’m probably answering my own question, but anyone with prior experience have any tips?


Finally had some time to look at things. Here is the latest and I need advice again!

Put the truck into 4WD and it moves forward and backward. The rear drive shaft spins. Put the truck into 2WD and it does not move. The rear drive shaft spins in this scenario as well and the speedometer registers speed.

Is this a rear end issue now? Do I pull the differential cover off and see what’s happening inside there? If so, what should I be looking for? Anything else I should be looking at or for?


So you are getting power to the rear differential, the input gadget on the rear diff is spinning, but the rear wheels aren’t rotating?

Definitely sounds like a problem in the rear end somewhere or another. Whatever it is, it wouldn’t usually happen all of a sudden, you’d hear various noises first. Did you hear any weird noises from the rear area in the months before it stopped moving in 2wd completely?

If this happened on my 4wd Ford truck, first thing I’d do is poke a magnet into the diff case and see if I retrieved any broken pieces of metal, next I’d remove both rear axle shafts and see what the axle end-splines looked like. If they looked ok, I’d remove the third member for a visual bench inspection.

George. We don’t recall any noises before the truck became laid up. My plan for tomorrow is to lift the rear end, spin the wheels to see how they react, and get into the differential. Thanks for your time and suggestions!