Bed Shaking(Truck Bed) HaHa!!

[BradMph]

Maybe it's time to repave your driveway,
Check for any kind of cupping on the tires? Are they centered on the hubs. The slightest out of center can do this. Aftermarket rims and undersized lugs can cause havoc just like this.
Wheel bearings ok? lift truck and pull and push wheel for play checking.

Camoit, wouldn't it be better if he jumped into the bed and took a ride on the bronco billy truck?

To test the driveshaft, going down hill take it out of gear and see if the truck still vibrates. This will remove driveline issues if it doesn't stop.

[1949panrider]

The truck is factory 1988 MM with factory wheels. It is a one piece drive shaft. I can look back at the left bed rail at 47 mph and watch it shake. It does not change a thing if I knock it out of gear on a down hill. Sometimes if I don't drive the truck for a few days it will shake. then sometimes it will be just so smooth at any speed. I bought these tires that are on it when I got the truck the first day so I don't know if it done the shaking with the old tires or not. I have been back to the shop I bought the tires from twice and he has checked the balance but I still think it is a tire.

[1949panrider]

I forgot to add that when I jacked the rear of the truck up and put jack stand on each side of the axle housing and run the truck up to where it should shake it did not, so that tells me it has to be the left rear tire even though it checks balance.