On my 01 2500HD 8.1L and Alison 5 spd I had the same problem. I was under the truck everyday for almost a week before I found the problem. It turned out to be a combination of problems. 1) The Carrier bearing was out- this was hard to see on the basic shift from park/neutral into reverse. I had a buddy ( that was more comfortable mashing the go peddle harder while I was under the truck ) hold the brake and work the gear lever and the throttle. That was when I could see the movement in the carrier. Reach up and move it by hand and you WILL see it move if it is out. ** This is when you will also check the lubrication on the splines and yolks of the shafts.** 2) The other issue is a problem the Chevrolet Knows about and insists that will not damage the differential as long as the proper maintenance is done at 15k-20k regular intervals. This problem ( according to my local Chevy house service manager ) comes from the input shaft at the pinion gear. He suggested that I check the torque on the pinion yolk nut and replace the seal and grease as needed. Bankston Chevrolet in Ft Worth, TX 76180

 

my truck clunks when i got into reverse on a daily basis (01 silverado 1500 z71) and i've changed the u joints already and checked the tranny fluid level but it still seems to do it

 

IF you do the procedure yourself be sure to use the proper lube as the lubricant has to be compatible with the transmission hydraulic fluid otherwise it could contaminate the fluid....
Here is a link to GM Part Direct for the lube that is recommended by GM
GM PART # 12345879
http://www.gmpartsdirect.com/results.cfm

 

everything came out good i just replaced the u-joints and re-lubed the yoke. the clunk sound went away. truck sounds good . thanks everyone. and good luck to the rest of you with the same problem.

 

did you ever figure out what was causin it to do that? my truck is also doing that and i cant figure it out

 

hey how would I check if my drive shaft splines are dry? I have a clunk about half the time when i come to a stop.

i also have a hard shift out of park only about half the time even on flat ground.

 

spray lube on it
see if clunk goes away.