Just finished looking at various drive line vibration issues in the search function. Although I have some similar issues let me tell my present problem. 1989 Volvo 740GL with 365K miles on it. Recently noticed a thumping in the drive train so looking back at my records found I had done a drive line rebuild at around 200K miles. So I put the car up on my lift and found a broken transmission mount. While there I pulled the drive shaft and felt the center bearing was a little rough so I did a drive shaft rebuild. New U-joints (spicer) along with center bearing and rubber and also a new tailshaft bushing and seal. Marked spline in DS but when looking at it , it can only go back 1 way . Installed everything and now have vibration at 30-50 mph range or around 1500-2000 rpm in TC lock up. Pulled DS and took it to a drive line specialist who checked it out and couldn't find a problem. Re installed and have same issue. Motor mounts are good so now now I'm going to pull the wheels and try a run on jack stands to see if they are contributing to the issue. I also noticed that the 3 u-joints don't line up where the front and center joints are aligned but the rear is out of phase. I don't know if this is a problem as this is the way it was assembled at the factory. I am also looking for someone local to see if they can balance the assembly but no luck so far. By the way at 30 mph there is almost no vibration . I'll have to check rear wheel bearings as well as the diff. but just tossing up my experience to see if there is something I missed.

 

I like your idea to run it on jack stands and see if the problem persists. If it doesn't, I'd look at your tires first. Bad tires have been the bane of my existence! Does the problem only happen in TC lock up? Sheared torque converter bolt or lost weight on your flex plate could be contributing. I hope you find your problem.

 

I don't know what you mean that the rear is out of phase but all three u-joints should line up. Should be like in this picture:

https://www.google.com/search?q=volv...NqoOo6zRaNM%3A

 

As an update I put the car on jack stands and removed the rear wheels. Ran it up and still have the vibration. It starts at 30mph and gets a little worse at 40-45mph. Doesn't seem to be RPM related so I'm still looking at the driveshaft as being the issue. Will talk to a driveshaft guy next week who also is a Volvo mechanic. I still am studying the u-joint alignment as it appears the rear joint is not in line but the spline is keyed and doesn't allow you to insert it in wrong. The factory used an epoxy on a few splines to prevent you from installing it wrong. I still need to maybe take it apart and re-look at this. I'll keep you posted.