If that was my car, I would check the valve adjustment clearerance. If upon seeing that is ok, with the engine running at idle, loosen the fuel lines at the injector one at a time and observe if any one doesn't affect the shake. If while loosening any one, there is little or no change to the shake, that cyl. has some issue with performance. If I found one that seemed week, I would pull that injector and check it for a good spray pattern. If you don't have the equipment to check injectors, just pull another one and swap the two. Do the loosen-the-lines test again and see if the problem followed the injector; if so,have the injector repaired. If all this fails to reveal anything, do a compression test. If #1 proves to have much more compression pressure, while the others are still acceptable, than I would have an explanation for the shake and drive the car for another 100 or 200 or 300K. That's what I love about these diesel cars. Milage is referenced by 100Ks. Again, I enjoy reading this forum. It really helps me sort out a lot of problems with my cars; as I am kinda new to the MB product.