I have had many posts about the clunking from????? or what sounds like my rear brakes letting go when they are stuck. I first replaced the rear brake pads and rear parking brake pads, about 6 months ago, That did not resolve the "clunk". 2 months ago I replaced both rear driveshaft ujoints. The did need to be replaced but this did not solve the "clunk". Last week my "service 4WD" light came on. I changed the transfer case fluid with the new "blue" GM Automatic transfer case fluid II. I now noticed that the "clunk" is now gone. A fellow mechanic told me that there was probably a build up of old fluid that turned to a hard grease that flowed thru the transfer case into the drive shaft and this was binding and not letting the rear driveshaft move properly. I think that he is right. Well anyway the "clunk" is gone.

99 Silverado 4X4 Lt ext. Cab......103,000 miles....hope this helps .....