I just had the u-joint at the transmission, the u-joint at the rear end, the driveshaft support bearing, & the rear pinion seal changed on my 88 Dakota 3.9 2WD changed yesterday. Last night, I noticed while driving at 60-65MPH, there was a vibration.
I was told that maybe the driveshaft was "unbalance". Any suggestions?

Was the vibration present before the parts were changed?

I really hate two-piece driveshafts...... they can be a pain to get installed properly, so you DON'T get that vibration.

After the parts were changed

In that case, I would go back to the shop that did the work, and have them take a look.

My mechanic is saying its a bad transmission mount. The transmission mount is causing the driveshaft to not be properly aligned.

Possible. Relatively cheap in the over all scheme of things..... Worth a shot I suppose.

Had the transmission mount changed and I changed the left rear tire. There is still a vibration when you get up to 55MPH and over. Any other places that I need to look?

Feel it thru the steering wheel, or the seat? rotate the tires, still the same?

Feeling it thru the seat. I may see about getting the tires rotate & balanced.