Sounds like a driveline viberation. Check to see if the pinion is loose on the rear end.
Also chect to see if the phase is correct on the drive shaft. If you were to lay the shaft on a flat surface the flatts on the inside joint holders should sit flat and not 90* to each other.
If you put jack stands under the axle, removed the tires and put the lug nuts back on the drums. Spin it up to speed and see if the truck still shakes. If it does it is a driveline problem If it don't it is in the tires.
Seems to be more noticable at lower speeds and it vibrates the whole truck.
Check the tires and make sure you don't have one with the cords slipping. It will cause it to have a bulge or be egg shaped.
