Drive shaft mount or flex disk? Either could b bad. Check the mount by shaking the drive shaft -- if you can move it in the mount, the rubber is bad.
The flex disk goes between the driveshaft and the transmission (or diff, but that one rarely goes bad) -- and signs of distortion or cracking means replace it, the drive shaft can come loose if the centering sleeve goes , too.
If the flex disk is bad and you have thumping noises while driving, get a new centering sleeve, too, it will be broken and allow the driveshaft to move too much. Get the grease that goes in it, too!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!