Originally Posted by clacker

No check engine light? The dealer will find lots of stuff to fix on the way to solving the problem as the car is ten years old, just what it is. Worn parts are worn parts. You don't want to go down the replacing injector path if possible, that will cost a ton of money. Have them run a cylinder balance test and find out which cylinder is the most out, then swap injectors around and see if it follows the injectors or not. ($2000 for a set?). Always install new washers under the injector. Ream if possible the bores and make sure they are nice and clean. Same for glow plugs, they have a history of being a problem.
Vibrations on diesels are almost never caused by a weak mount, the vibration wore out the mount (meaning the mount alone is seldom the source of the problem). Weak sensors, worn injectors, internal engine issues can all cause vibrations.
I don't have first hand experience with that engine (OM648) to know what is common, but I doubt the serpentine belt would cause much vibration. More likely a shut down valve or shutter in the intake manifold is not working properly to prevent a shake when turning off the engine, these do make a big difference (but not while running) unless it is sticking from carbon build up. Maybe the intake is getting clogged up, have a peak at the easy to remove areas.