have you tried this without rims/tires on the back? if you do try this, make sure you have a couple of wheel nuts each side holding the rotor tight....
did this start to happen after you changed pinion seal?
watched a guy with a 40 ft motorhome ram the back bumper of my truck(twice)....:rant:rant:rant(someone almost got a beating....:D)
i guess i will have to add it to the windsheild that i have to replace....