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I've seen this problem before on Dodge vans. If the truck is lifted on a single post lift, when the rear end drops from leaving the ground, the driveshaft will bend against the edge of the lift. Examine the driveshaft carefully for a small crease in the tube near the front. If you find one, that's what happened.
Also, go back over all the u-joints and look close for centering of the yokes. Make sure all the lock clips are in place and none are bent or jammed in.
You've probably already done this but jack up the rear end and run it to examine how true the wheels and driveshaft are running
Sorry about the neon thing. I don't know how that happened . I was looking for that.
You stated you had the driveshaft redone. I take that as you had yours repaired. If that's the case, you still haven't determined if yours is damaged or if the joints were installed properly. Differential bearings tend to make a lot more noise that vibration unless it's the pinion bearing letting the driveshaft whip but that would be pretty obvious when you shook the shaft.
You CAN"T straighten a driveshaft. It is a balanced component. Why don't you see is you can pick up a used shaft or even borrow one from another van just to try out. You may be just running yourself in circles, ignoring the obvious. I have seen many of them bent accidently when the truck is lifted incorrectly and the damage is vey difficult to see with the naked eye. You have to look for a very small dent in the tube. If you find thast small dent, you be sure that shaft is junk.
A bent axle will be completely obvious when the van is lifted and the wheels are spun. You really can't miss that. There shouldn't be any question about them.
You may actually need to have it balanced by a drivetrain shop.There is a tool called an Electronic vibration analyzer that will let you balance the driveshaft,and pinion assembly on the vehicle withe wheels and drums removed.Is this vibration so severe that he can't drive the van?