My 840 used to vibrate between 20-25 mph then the vibration would go away. It turn out to be the cv joints. I replaced the whole drive shaft also ($700 for the part, installation took less than 1/2 hr.
You're better off by replacing the whole drive shaft if you can afford it.
update, diff and subframe bushings replaced, car dives much better, but therre is still minor vib between 15-35mph and feels like it is comming from driveline (somwher in front)
all suspension have been replaced, guibo, csb,engine and tranny mounts, spark plugs , valve cover, prytt much everything that coud cause vibrations;
any ideas?, forgot to mention - new wheels anf tires (raod forced balanced)