Sorry to see this so late. I have owned a 300D for 30 years and they are a little different
than other years because they are a crossover from the 1970's to the 1981 and beyond.

My 1980 300D non turbo had the exact same shifting problem and it was the vacuum
valve on the valve cover the 3/2 valve. After replacing it with parts purchased here
it solved all my problems and I learned how important the little replacement arms are that are also sold here (so you do not have to purchase the entire valve as the arms wear out.
But the valves do leak, and that causes harsh shifting.

So my point is, the 3/2 valve on a 1980 is important for shifting. Do fix it.

On other years, those valve cover mounted 3/2 valves can be removed.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250