How much, if any lift are you running? First I'd get a magnetic angle gauge from the hardware store and measure your u-joint angles. If they are over 7 degrees you may get the coil springs to vibrate at certain times-usually on float when you're not on the gas, but not decelerating either. It sounds and feels like hitting the rumble strip on the side of the highway. If that looks ok and it seems like it could be one of the gearboxes, I'd try to narrow it down a bit further. Does it do it the same in all gears and reverse? Does it do it the same in low range? is it related to road speed? Is there metallic particles in the oil? Early LT77s have really small bearings on the countershaft that will fail and cause the shaft to shift. What are the serial # on your gearboxes? What engine is in your truck?