Vibration

It could be damaged rims. If you move the tire to the front andsee if it changes then it could give you a hint. But if you put the rim on a balance machine an dspin it you shold be able to tell if it's bent or not. Youi could also have had a weight fall off another tingto check is the pinion where the driveshaft bolts to the rear end as they can be out of round as well you can 'try a fwheel balance on the truck as it may be a bad drbrake drum if it's got drum brakes as well, they have weights as well and one of those may have fallen off