You know, that makes sense since it happens when he lifts his foot off the throttle which would make the weight transfer back to the front and off the rear... Hence the clunk... Ya, try to tighten that up and give it a shot. Sounds like the dealership is just skirting the issue... Much better to have suspension a bit loose than tranny backlash or tranny issues. If the clunk happens when power shifting or downshifting, then that would be another issue.