Somethig else to do while you are checking the Shifter.
I would be curious and drain the Fluid and inspect for anyting in the bottom of the Pan. If you find stuff in the Pan that could be a clue as to what is going on.

If I did not find anything in the Pan I would try changing the Transmission Filter. You could reuse the ATF if you wanted to to avoid further expense in case the Filter Change did not help.
I fact if you decided to get another good used Transmission you could remove the newish Filter and put it in a Zip-lock bag and reuse it later.

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84 300D, 82 Volvo 244Gl Diesel