Here is a great post with transmission adjustment information:
http://www.peachparts.com/shopforum/diesel-discussion/158216-its-critical-how-you-set-your-transmissions-vacuum-system-your-diesel-mbz.html
As per the thread, it is very important to get the correct vacuum diagram for your vehicle to ensure that correct vacuum is applied to the transmission through a special orifice (If the vacuum lines have been "modified" this orifice may even be missing).
On the MB diesel engines, the vacuum modulator is set by internal pressure of the transmission, which is checked by a test port and pressure guage, and is specific to your model.
The problem we run into with the MB diesels, is that unknowing mechanics try to adjust them the way you would a gasser. A typical scenario is someone gets an oil change and then complains of hard shifts. Rather than find the vacuum leak caused by messing around in the oil filter area, they dive in and swap lines around or adjust the modulator. Once the modulator is set per the pressure requirements, rarely should it ever need another adjustment.
If the transmission kicks down with no vacuum applied to the modulator, then I would tend to lean towards a problem with the vacuum system. I know that the link provided is a long post but well worth studying.