check that bushing on the "bowden rod" that connects on the transmission end, mine was gone and resulted in some funky shifting like you described. all's well after replacing that bushing

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL