I have a vibration at 35 mph and up. I have narrowed it down to the rear of the truck. I jacked up the back and put in drive and the vibration started once I got the rpms up. I then tested each side separately by lowering one side at a time and doing the same test and got the same results with both sides but with a slight grinding noise. I have also just recently balanced all the tires.

 

Perhaps to help narrow things down, secure the truck on safety stands, remove the wheels, ensure the rear brake assembly isn't going anywhere, and take it up to speed.

Still vibrating?

Might need to look hard at the driveshaft. If you suspect it, I'll see about posting the FSM steps to sort the driveshaft (too long to type).

EDIT: Check the spring U-bolts and be sure they're good-n-tight.

 

Also might want to look a the driveline flange on the back of the transmission, if it isn't a slip-in type. Or the flange on the differential where the u joint attaches. I have had vibration troubles on several trucks due to one of those flanges being loose.

Of course, that assumes that you've already checked out the u joints themselves.

 

Sometimes it gets so violent that I feel like it is going to shake me off the road. If that helps anyone with their diagnosis.

 

I went through the same thing I found two problems I jacked up the pickup to check the axle bearings and found the breaks were bad and the bearings were bad so I ordered bearings and breaks and new drums for both sides thought I would fix the problem but it seemed worse when I got done. So then I went after the ring gear and pinion and side bearings found that they were shot to so I replaced them but that did not fix the vibration at all. I was really frustrated now, but I did find the problem it was the new after market drums I put on. My problem was a combination of two things one was the bad bearings and the aftermarket drums I put on. So never buy aftermarket drums buy Mopar. Forgot to mention I own a 1993 D350 Dually.Good luck.

 

Have you looked at your driveshaft and u joints closely yet?