I've had this problem for years with my '46 1/2 ton. Do you have the 3 speed or the 4 speed? Couple of things to check. 1st, try to isolate the problem to driveline or engine. When the vibration gets going at road speed, what happens when you step on the clutch? Is it worse or better? A bad motor mount can make the engine "ground out" on the frame with metal to metal contact. If that's it, the vibration will lessen a lot with the engine idleing while you're moving down the road at speed. Another thing that can cause engine vibration is an unbalanced flywheel.
I've got the heavy duty 4 speed and I've found that the U joint gets inadequate lubrication. I pull the pipe tap on the top of the rear trans and put in about 4 oz of 140w straight weight gear lube every time I change the oil. This helps with the vibration and greatly prolongs the life of the U joint.
It's a vexing problem. Let me know if you solve it. Good luck
David Bush
