What year 300D are we talking about? If its the 81 in your sig, that has a 722.3xx with no external adjustment.
The clutches are most likely worn out. Out of the three 722.3xx tranny's I have had apart all three had reverse(b3) clutches worn much worse than the other clutches. All between 210,000 and 240,000 miles only one was still working correctly but worn out.

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81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)