Well, with 150k miles on the frame, it doesn't surprise me that you have body mount issues. It won't be too hard, but it would take a while to change out the polyurethane mounts. I would go and try that first off and see if it takes care of the problem...if not--oh well, the body needed it anyway. If that doesn't solve the issue, I would look at going to get the frame tweaked like Greg said...or go and have the leaf springs rebent or put some new ones on...