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.