The 3 things that I was always taught to check are:
1) Motor mounts...they can cause a LOT of vibration and shaking on a diesel
2) Try adjusting the rack dampening screw on the back of the fuel injection pump. This can smooth out a lot of shaking.
3) try running a can of "diesel purge" through the fuel injection system. If the fuel injectors are dirty that can cause problems as well.
You might also check to see if you need to REPLACE the rack dampening screw...when they get worn the idle tends to get pretty bad
Also check the idle speed...if too low it can cause vibration and make the car seem as if it's getting ready to stall out.
Good Luck!
