clunk after shift into 4th gear

[sgbaker]

so my 98 tahoe has a weird issue and so far mechanic and transmission shops have to clue. at speeds above 45 mph i get a clunk about 3-4 seconds after it shifts into 4th gear. replaced u-joints, had transmission serviced. rear diff serviced......no change. trans shop says motor, mechanic says transmission.
really tired of throwing money at this problem.
any ideas??