rear knock on '01 2500

I'm at aloss to figure out a knock in the rear end. Usually when good and warmed up I get a knocking in the rear end. It ONLY happens when coasting and occurs in a straight line or turning. It feels and sounds like a hub, which is what I thouht it was till I brought it in to a fwd shop. They it to do it but could not find anything. They said the rear had been overfilled, did a "posi service" and refilled. Somethimes it doesn't do it (usualy if there is a mechanic nearby) but I can usually get it do do it aftre a few hours of high speed driving. When really bad it will do it right down to a stop and when rolling very slowly you can feel it binding as it knocks. A friend seems to recall burbs having problems with rear bearings going ot of round but it seems like if that ws the problem it would do it all the time. Any advice greatly appreciated.

I won't happen all the time yet because of two factors you mention. The several hours time factor means something is warming up before it will 'thump'. A bearing takes some effort to warm up especially in New England this time of year. Coasting means it only happens when there is no load on the power train. I think you need a real close inspection of the drive pinion and ring gear or other 'posi' gears before you need a whole new rear end. Things that make this much noise usually don't just go away or fix themselves.

This sounds nuts but I have a '99 2500 454 4WD which had a subtle intermittent knock from the rear of the vehicle. Same symptoms, I swore it was drivetrain. It turned out to be the exhaust pipe knocking against one of the frame crossmembers. Stock exhaust, very close fit. I had an exhaust shop pull the pipe and tweak it a little. Works great. Could be?