http://www.vbandp.com/SearchResult.aspx?CategoryID=42

try these people

kickstart

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Sounds to me that the roter is slightly warped.... I honestly have
very little faith in most dealers when it comes to these  problems...

My right rear rotor was out by 27  thousands when I noticed a similar
problem a few years ago....replaced it with a new roter (out by 7
thousands when installed BTW) and the problem went away....

Thats my guess...

Bob G.
64 72 76 79 & 95 Corvettes...
 

Also check for worn/loose bushings on the trailing arms and differential
case.

There is so much stuff that can move back there, this description is like
saying "My head hurts."  Well, does it hurt from a headache, you slept
wrong, you have a nasty pimple, or your wife smacked you upside the head
with a club for being out too late drinking?

Those "thumps" could be clicks, clunks, pops, and a host of other noises.
You could have pads clicking because they stick to the rotor because they
are worn and the rotors glazed, you could have u-joints popping because they
have play, you could have noises in the rear end because the stub axles are
worn, you could hear Posi clutches popping and snapping, and more.

Heck, my '79 has popping sounds all the time.  Just recently, I had some
that turned out to be a real nasty, or could have been very nasty, problem.
I put a set of old aluminum slots on for that retro look and I picked up the
unilug inserts from Pep Boys. The original inserts when in the back and were
thick from Ansen. The only ones available now are cheap washers and these
bent, loosening the wheel while I was driving (after a month of having them
on), and I snapped a stud the other day. I had kept checking tightness but
the nuts had finally bottomed against the rotor, and only the lock was truly
tightening.

So you have thumping. Yu need to narrow it down a bit.