Dale Traveling is probably correct suspecting the Ford chassis bottoming out. To check if this is happing, get some kids modeling clay, form it to the bump stop snubber with tape, and go for a ride. You will probably find the clay completely mashed-out when you get back and check. Contrary to popular belief, Ford made their springs very soft, for a better ride, which allows the bottoming to cause a harsh ride. This is also why a lot of Ford chassis MHs lean to the passenger side.