psaboic,

Pictures (front/back) of my front flex disk below. Even in this condition, there was no slop in the disk. In fact, even with total loss of the driveshaft support, the car was running pretty quietly. There was the low-frequency hum at certain speeds, indicating something clearly wrong, but nothing like the thumping through the floor you describe... The rear disk looked nearly perfect.

I'm definitely with Stretch on checking your motor mounts at this point. This would both put your drivetrain out of alignment, and could account for the drivetrain gettin' a little closer to the floor than it should be! Might also put your exhaust system in a new position!

So also inspect exhaust. When my header pipe broke (above the mount U-bracket), the header-resonator joint was forced upward, occasionally vibrating/thumping against the driveshaft tunnel. Happened only at certain speeds, too, when the frequencies were just right.

For point of reference: These flex disks have been in service for at least the 200k+ miles I've owned the car. Likely they're the '83 originals... 400k+ Also, the U-joint showed no signs of wear; I could not produce even the slightest click or play in the joint. (Should I have replaced it just for good measure?)

Attached Thumbnails

   

__________________
'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr