Sounds like a linkage problem to me. I had a shift arm come off the shaft and could only use 3rd and 4th gear. It made driving home through stoplights interesting.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine