Could be lots of things like everyone said...

Could be the tires out of balance...

Does the wheels have curb checks?

Could be the DS...

Could be the U-joints...

Like the other said - Something about the Wing and Trunk loose? (I've never heard of this, but its an interesting possibility)

I also read something about turning the DS 180 degree at the yoke ....

But in order of least expensive...

Check the trunk,
Check the weights on the rear wheels are still there..
Check the wheel for really bad curb checks..
Check the tires for separating cords (maybe)
Check the U-Joint (cheap to replace it - $20 from autozone but pita to replace)
Then the DS
Then the rear end or Tranny.....

But Tranny should only be checked if the vibration goes away if you put the car in neutral at 90 mph....

Ah yes, the search for that evil highspeed vibration... Its freaking PITA..

 

Buy an aluminum driveshaft and the problem should go away. With larger gearing the driveshaft will get out of balance and play havic at higher speeds. I replaced my stock driveshaft two years ago on my Impala with a Inland Empire aluminum driveshaft for 324.00 from Summit and have made aleast 100 trips down the drag strip and still no problems at high speeds. Good luck
HVY SS