Rear end thunk

Hi,
I have a 89 checy C1500 2wd. Sometimes when I stop I get a thunk that sounds like its coming from the rear end. The sounds also occurs from taking off as well (sometimes). The rear brakes have been completely re done and just put in an new drive shaft and u joints. I checked the leaf springs and none are cracked or broken but the bushings are quite dry, I dont see any wearing on the axle. Any Suggestions would be greatly appreciated.