1987 740 Turbo 4speed manual w/overdrive

Here are my symptoms :
- at around 50-60 km/hr a groan begins....more like a low waa, waaa, waaa
- with increasing road speed the frequency of the waa, waa, waa increases until around 100km/hr it is just a constant low rumble
- sound is most prominent in 3rd and 4th between 60-80 km/hr (epsecially if the motor is bogged down a bit going up a hill)
- the noise is worse, the colder it gets
- the noise is road speed dependant...not RPM related, but is a bit more noticable when the clutch is engaged
- the noise/rumble feels like it is below the console

Here is what I know....
- had the U-joints looked at..the shop says they are fine (at the time I did not ask them to look at the bearing)
- changed the tranny mount since it was pretty beat-up (note that this noise became more apparent shortly after changing the mount, which also increase the ride height of the tranny by about 1/2"
- tailshaft bushing looks ok and there are no leaks while I was under there
- fluid levels in the tranny and diff are ok

Is this my center driveline bearing/mount? Unbalanced driveshaft? Combo?

I have crawled under and looked for play in the rubber mount. How much is acceptable? 1/2" is what I was getting?

I have a parts car. I may look at just switching the driveshafts and bearings/mount?

Any thoughts?

Any risk in just switching the shaft in from another car? If it is balanced it should just go right in should it not?

Thanks