Vibration under power, (harmonic damper?)

[HHhuntitall]

As it was mentioned that it happened only under power, and with it being an automatic, having checked the U-joints, I'd suspect, as cerberusiam said, it's likely a tire or torque convertor. While the bolts may be tight in the convertor, there could be a crack in the flex plate, which would possibly only show up under torque/power. With your mention that it has moved to the rear of the truck, it could very easily be a tire..... Keep looking.