A couple things I can think of to check-- run out on rear wheels while on the truck, which could be caused by bent wheel, wheel not seating square on hub, bent axle or flange. Only other thing I could suggest would be to have the wheel balance checked on the vehicle if you can find someone that has the equipment. My first thought as I started reading your post was a bent drive shaft because I had a car once with the same problem you discribe but if the balance has been checked then I guess you are good there.
Good luck
Dave
