Reading the posts I would check a couple of things.
On the 240D check the motor mounts and the transmission mount. If they are old and collapsed they will affect the alignment of the drive shaft vs. flywheel. Check the front engine stop and the tranny mount. Again if these are bad they affect the position of the flywheel and the output to the transmission/driveshaft.
Check the drive shaft center bearing and rubber sleeve. The center bearing/sleeve could allow a repositioning of the driveshaft.
Check the rear differential mount. Is it broken, againg allowing the driveshaft to not be aligned correctly.
If all of these items are OK then the only other thing I could think of is was it a OEM flexdisk? or after market?

http://skinnerbox.steaky.org/Service/W123/Index/616index.html

jobs

00-211 - Removal and installation of front engine mounts

00-212 - Removal and installation of rear engine mount

00-215 - Removal and installation of front engine stop

00-220 - Adjustment of engine stop

then

Check all of these job items

http://skinnerbox.steaky.org/Service/W123/Index/Groups/41PropellerShaft.htm

Again you may find the other components of the engine/tranny/driveshaft/rear axel support system are your problems.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car