1994 Ram 2500 pickup 4x4 auto

My truck started exhibiting a strange new behavior, a vibration that comes and goes. It happens only under a certain specific condition; it depends on how much fuel I'm giving it. If I'm accelerating or coasting, no vibrations. But if I'm giving it just a little bit of fuel, maybe enough to just barely maintain speed on a level surface, it sets up a noticeable vibration. Not shaking the whole truck, but enough to be noticed.
Apparently the vibration is a function of how much torque is being applied to the driveline. I experimented by driving along and feathering the fuel pedal; there's a rather narrow range where it sets up the vibration. When I give it a bit more fuel or let off and coast, the vibration goes away.
I speculate something in the driveline, somewhere between the transmission and the rear wheel bearings. Something loose or damaged. Applying torque on the components apparently squeezes out lash or something, or whatever causes the shaking.
Looking for ideas. Has anybody else ever had this happen?

 

u joints a possiblity or carrier bearing or out of balance shaft get some big hoseclamps and clamp washer to drive line and movearound postion and see if gets better if so its out of balance but sounds like ujoint or bearing problem