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Hey guys, that clunk that you are hearing is not the differential. all z71s or 4x4 have this problem. its the yoke that enters the transmission on the rear just sand it down with 220 grid sand paper on the outside and inside, the splines tend to get surface rust alot. after you do that put some hi temp grease inside the yoke install it and there you go. the noise stops
It could be as simple as the exhaust bumping the rear end. Or the trailer ball hitch rattling inside the receiver! (I had one customer wanted me to weld it so it wouldn't rattle any more!)
Slide under there, don't jack up the truck it will change the normal ride clearance. There should be a few inches of clearance between the exhaust pipe and the rear end. Move it around see if it bumps the rear end or the frame.
It could also be a bad U joint, grab the drive shaft near the U joints and move it around, if either is bad you'll see it moving. The caps that connect to the drive shaft have needle bearings inside them, if they're bad it will move or clunk at the cap, near the rubber seal.
Or it could be a little excessive end play in the rear end. Some cars/trucks are noisier than others. USUALLY not a big issue. It could clank for years. Just annoying.
I would suggest a rear differential service. When the fluid breaks down in the rear it doesn't provide the proper lubrication for the rear and you'll hear the "moan". Don't forget to buy and add the limited slip(posi-trac) additive.
As far as the rear end noise, There was a TSB released on GMC light duty trucks that says
"Some owners of light duty trucks equipped with automatic transmissions may comment that the vehicle exhibits a clunk noise when ******* between Park and Drive, Park and Reverse, or Drive and Reverse... Whenever there are two or more gears interactin with one another, there must be a certain amount of clearance between those gears in order for the gears to operate properly. This clearance of freeplay (also known as lash) can translate into a clunk noise whenever the gear is loaded and unloaded quickly, or whenever the direction of rotation is reversed."
It goes onto say "...Whilse some owners may find the clunk objectionable, this will not adversely affect durability or performance."
The noise is caused by the axle button being worn out, this lets the axle move back and forth and clunk, this is a unique problem with C clip retained axles, it is a noise issue but will not fail and cause any more damage, in order to fix it you must have the axles pulled and the diff carrier checked for excess wear where the axle retainning C clip rides, if the carrier is ok then replace the axles.