If the setup on the 300D is the same as my 80 300SD then I would first check the bowden cable (actually it is a rod on my car not a cable) and see if the rod is missing the bushing where it attaches to the lever on the passenger side of the transmission housing (toward the front of the transmission). My bushing was completely gone and the lever was not operating properly. It was causing the transmission to start off in 2nd gear and shift very early from 3rd to 4th. If all is tight and intact then move on to the vacuum part of the system.

__________________
Jeremy
1979 300SD
1980 300SD (parts car)
1982 300SD (WVO conversion Frybrid Kit) SOLD