Is the engine out of balance (flywheel, crank?) or something in the transmission (torque convertor?) out of balance? I'm no truck mechanic, but I have dealt with finding various vibrations in equipment and large rotating shafts. It can be a pain tracking them down when all the obvious components have been inspected or replaced and very frustrating.
Did you get to drive the truck much before you did a bunch of work to convert it to your dump truck? I remember it was an old firetruck, did it have a vibration back then? Also, when discussing natural harmonics, increasing the weight (mass) of an object changes its natural resonance and can get rid of a vibration at the desired rpm range. So, if your driveshaft is too light for the specific design length, it might shift the vibration to a different frequency if the driveshaft is slightly heavier. Did anyone ever chuck the driveshaft up in a lathe and check runout along the shaft at various points to prove that it isn't slightly bent? That would certainly affect its natural harmonic as well. Just some thoughts.
I drove it as a fire truck maybe 70 miles total. Seth drove it another 50. We noticed none of the shake it now has. I believe the shake developed after alterations. We shortened the frame roughly a foot. The fire pump & the fire body were removed. Because the pump served as a sort of transfer case, it was inline of the driveshaft. A short shaft two universals & slip joint ran transmission to pump. A shaft ran through the pump, mated to another U joint, a short shaft reached the carrier bearing, then another u joint, then another slip spline. It ended at the pinion with the last U joint.
New drive shaft has U joint at rear of trans, a carrier bearing, another u joint, a spline, at pinion another U joint. Present shaft is three U joints a carrier bearing, and one slip joint. The foremost driveshaft shop replaced three U joints, a carrier bearing, cut the yoke off & corrected out of center. They then balanced the whole assembly.
Trying to correct the shake the whole truck got new wheels & tires. The 10,000 mile brake drums have been checked for run out. Tires have been back for rebalance. Rear spring equipment down to bolts is 100% new.
Spicer says U joint at transmission needs at least 1 degree, not more than 1-1/2 Degree angle. The angle of front section of shaft must match pinion. None of this seems to be more than a degree out of spec. New angle gauge is ordered.