My 85 did it and my 90 started doing it. I lifted them both and it stopped. Asked around and got two answers from about 5 different guys:
1. The poly urethane bushings in the leaf springs wear out or loose or broken shackles. When I put the lifts on the Burbans I put in new bushings and it stopped. (Didn't want to believe this one but when I lifted my 90, one of the front driver side shackles was completely sheared in two. As much as I crawled up under that front end and looked I never even saw it was broke until I took it all apart for the lift).
2. Was axle wrap (which I personally think was the cause). After installing the lifts and putting shims between the axles and springs it cut out the klunk.
Who knows if we will ever know? I say start with the cheapest fix first!
There are allot of better qualified gear heads in here that could probably pin point it.