1981 300 TD Wagon Transmission doesn't shift

While driving, the trany shifted to nuetral by itself. No position on the gear shift changes the condition. However, when I shift to park I do get a grinding noise before the gears mesh. Every other position is quiet. I was told that I may have to replace the transmission and I am willing to do so as long as a second opinion agrees with the first.

If I do need to replace the transmission, I need a good manual to help out.