Fathead,

Some of these screen names kind of set you back a pace when you respond....

It sounds more like an idle speed instability than a transmission issue. When you put it in neutral you take the load of running against the transmission (pump, fluid shear) off the engine. It is likely the engine speeds up a little to its normal idle speed, and then when you put it back in Drive it slows down and gets a little rough.

The idle speed on later Diesels is not manually controllable like it was on W123 240D's and some of the other models. I am not familiar with how this is done, but I am certain I have read threads on this board with the procedure listed. You might try a search on rough idle and see what you get.

You might also have some worn engine mounts. As the engine is loaded slightly the torque causes the mounts to be loaded. If one or two are bad, the mounts can transmit more vibration, and actually set up a resonant response to the engine, exagerating the engine vibration at that frequency. Most of the time mounts that are tired are visually apparent. They are cracked and flattened. In most cases the passenger side mount takes the compression loading of the torque reaction, while the driver's side will see cyclic tension and compression loads. This can cause the driver's side mount to fail early and more dramatically, even shearing all the way through.

Hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)