Originally Posted by S-Class Guru

I agree; driveshaft. Most likely suspect is center bearing support as stated.
the driveshaft front flex couplers can get pretty bad and still not vibrate, they just clank a lot.
You didn't state if the vibration is felt solely in the steering wheel, or in the body. If it's the latter, my vote goes up to 90% for the evil driveshaft center bearing. These are really not that hard to repair, once you drop the entire exhaust of course.
Check the U-joint just behind the center bearing also. These usually have a pretty good "detent" in them, but hang on for a long time with no bad habits.

DG