Dad would always use a dial indicator that would magnetize to the body/frame to check if wheels were true/out of round. Did you have the truck on stands, and jack the center of the axle up to it's normal "driving" position? I like the idea of having a spotter with you(on the road or on the garage) or have them in the truck, and you be the spotter. Your spotter could tell if one side is hopping worse, and if the axle is tracking properly. A dry cup in one u-joint could also cause the vibration, or a mis-aligned drive shaft/axle. Good luck, guys.
