Wait, is it manual or auto?
If manual, and you just had the engine out to replace the oil pump, I'd check for binding on the shifter, first .
If auto, and this was a pre-existing condition, get a can of Trans-X at most auto parts places ($8). Follow the directions. It might only be a shift valve sticking. This will free it up. They get a lacquer build up on them that makes them stick. With 1000's of an inch tollorance, it doesn't take much to cause a problem, and that's why tranny fluid has so much cleaning chems in it.
You'll have to drive it around some, and get that stuff circulating good for it to do it's work.
If that doesn't free it up, you have a bigger problem.
Personally, I'd swap in an M50D as soon as I could.