Ok-

Does the truck make the vibration when in park or neutral, not going anywhere? If not, its not the harmonic balancer.

Only does it in gear, moving?

Next thing I'd try is put it back up on the jackstands, remove the rear shaft, run it in 2wd (front shaft shouldn't be spinning, or spinning very much- you should be able to keep it from spinning by tying a strap to it or something, or with your hand if you're careful. Or just remove both front and rear shafts.

If it doesn't vibrate in neutral then you eliminate the engine. Up on jackstands w/ no driveshafts spinning eliminates everthing after the transmission, so then its got to be something in the transmission/t-case. My guess would be the torque converter, simply because its probably the only thing big enough to make that much of a vibration.

OTOH-


This sounds an awful lot like an engine miss under load, maybe an ignition issue or maybe the timing is off. You would think the ECU would throw a code eventually though? Check the physical (belt) and ignition timing.