I have a 1991 D150 LE, 318 engine, A500 transmission, open rear end, 2 wheel drive and she is the definition of a good ole truck.
Recently I had noticed that my truck seemed to have a shimmy to it when I reached 45 mph. The vibration would possibly phase out and it kind of would come and go as intermittent problem. I proceeded to get on the highway one day and reached 60 mph and literally my truck started dancing, she was shaking and there were no good times happening in the cab, lol. I realized that this problem could have been caused by anything and so I decided to trouble shoot with some buddies. I decided to call a hail mary and replace the U joints, I made sure they were pressed in and perfect ( trust me the stock ones needed replaced anyways). Replacing the U joints actually kind created a new shimmy at 35 mph and the it shakes constantly now from that speed and on. Maybe coincidence that the problem may of gotten worse with the U joint replacement, I hope. So then we proceeded to get the rear end on jack stands and drive it to eliminate any possible problems in the front end. So I reached 15 mph and then I felt the shake from there. I removed passenger side tire, drove and still felt the vibration. Removed the driver side tire, drove it and still felt the vibration. went on to removing the brake drums and then decided to drive it with the tires and brake drums off the dang thing, and still the vibration ensues. So its not balljoints, tie rods, front wheel bearings, tires, or anything in the front end period. I dont see any places on the drive shaft where balancing weights could have came off. Anybody with suggestions and solutions will be given woman and beer for the rest of there life. Seriously thanks guys in advance.

 

I will move this to the 1st gen section (pre-1994 Dodge Rams) for you, get you some help.

 

You haven't left much to choose from. Could you have bent the drive shaft itself? Has your truck been stuck or hit something to bend it?

 

I agree with SEAL here, it's almost got to be the driveshaft at this point. It's the only thing big enough to cause that kind of dramatic vibration that you haven't eliminated.

 

could it be a bad trans mount? just a thought.