rear end slip?

First of all...I have a 99 silverado ext 4x4. I have read all ot the post about the rear end slip and transmission slips, but mine is a little different. I have 97,000 miles mostly highway miles and have recently noticed that sometimes when I brake hard and stop or when my truck is cold that as soon as I start to go again it feels like the rear brakes are stuck and are holding for a second or two then I hear a low clunk then all is fine again. This is very intermittant and is only when I brake hard for a red light or such. I recently had to replace my front rear ujoint, i did just do a total rear brakejob myself. All new brake pads, rotors, and parkink brake kit all from GM. (got the parts at cost from pal at parts). This noise was happening b4 i changed my brakes, and is still doing it. It actually feels like the body is moving, foreward, then returns to its normal position as soon as I resume again. (i do know that this is not the problem).
if I stop hard then ease into the gas, I do not feel the clunk. but if i get intop the gas I can really feel it. What does this sound like?? I have changed my tranny fluid at 55,000 miles and never changed out any other axle fluids. Other than this all is ok. If it is the rear end or tranny problem would this be intermittant? And how can I onfirm if it is the tranny of rear end???

I have an '04 Silverado... I am experiencing the same problem when stopping quick...it doesn't happen all the time. I would say it's happened about 6 or 7 times since I bought it brand new.
Is the normal? I wouldn't think so.. Any thoughts are greatly appreciated.

My '01 1500 xCab is doing it some times and I have 150k on it. My friend's '03 3500 cCab with 65k is doing it some times. I say dont worry about it deal with a problem when it becomes a safety issue.

If You Have Rear Drums Adjust Them The Shoes Are Being Pushed Out To Far
Adjust From The Backing Plate Or It Could Be The Rear Brake Pads Releaseing