7.3 2002 F350- when I take off or come to a stop (on occasion) I feel a pop or shift in the rear of the truck. Kind of like something is loose and shifting forward or backward. Seems to be coming from right side of rear- but not sure. I thought it might be U joints but was told I would feel vibrations driving which i don't think is happening, also had local shop look at it, and drive it, and they stated they didn't think it was that either, only that rear end was kind of loose, but that was common with these. Whatever this is though it doesn't feel normal. There is some damage to the bed- and wheel well's, wondering if maybe the bed is just loose and shifting???? Any Ideas??