Run a couple bottles of RedLine injector cleaner through it -- you may simply have varhished injectors that don't spray well at idle.
The gel filled mounts are a combination rubber mount and shock absorber -- this is why things are so smooth when they work well.
The easy way to check is to simply look. There is a tapered portion on the bottom -- if you cannot see it easily, or the rubber "ring" in the center, just below the steel hemisphere on top, is close to or sitting on the frame, the mount is shot. Ditto for any evidence of oily stuff (the polyethylene glycol) leaking out.
Check for vac leaks (bad vac lines are a possibility, also leaking idle control hoses), bad plug wires (do you have the funky dual plug wires? If so, they may be bad and the second plug doesn't fire right), etc.
Also check the tranny mount -- if it is too soft or separated, it can cause vibration problems 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!