Dave - Jerry had suggested when you do the one at a time rear axle check, try and pull the wheel towards you (away from the center of the truck) when that side is off the ground. If it moves outwards from the center of the axle, it's likely you have a broken axle shaft.
If neither wheel/axle pulls towards you from the center of the truck when you retest each side, the next logical step is to get the rear axle off the ground, drain the lubricant and remove the rear axle cover (10 bolts) and inspect the internals of the differential.
If nothing unusual found their, the next step is to tackle the torque tube as Jerry indicated.
Good luck sir!
