I bought a '82 SD with a bad trans. It would work ok for 10 minutes or so, then unless the car was not already in 4th gear it would not shift into it.
The explanation is that the clutch packs (this is not something I understand
but have read) leaked internally and the trans fluid was thick enough when cold but viscosity was reduced when warm and the leaks prevented shifting.
I did drive the car for a while like this. Added 3-4 quarts of lucas trans fluid
(which is like 90w trans fluid) and it seemed to go into 4th but still slipped a little, but driveable. I would not recommend the lucas fluid unless you have nothing to lose insofar as a transmission. Hopefully my scenerio is not related to yours, perhaps the problem can be fixed by adjusting the bowden cable or vacuum to the trans. Make sure the adjustment is right. I'm not an expert, so don't assume the worst yet.