[TDIGreg]

Are your tires catching the mud guards?

 

[TDIGreg]

I just know a guy that bought his truck new and ran stock tire size and it always did rub occasionally when he was backing up with the steering on a lock.

 

[TDIGreg]

Be sure to check to see if the front of the tire is catching the fender liner too. Is this truck leveled? If not, you could try leveling it and see what happens.

 

[TDIGreg]

Does the thump happen just once when you put it into reverse or does it thump, thump, thump with the tire revolutions? Does it only do it with the steering on a lock? Have you checked the torsion bars and when the were under load for any play?

 

[TDIGreg]

If you are 100% sure that the noise is the torsion bar clunking in the lower control arm, make sure that the rubber bushings are good and not split or cracked. If they are bad it could allow the control arm to rotate slightly and cause the torsion bar to thunk in the socket.

Also did you loosen the torsion bars when you lowered it? In other word are the torsion bars under tensioned?

I also attached a tsb for clunking while turning or driving on uneven ground.