How long has this tractor been sitting? I had a 420 JD  that had set several years before restoring the engine and one shift rail had been without oil so long the thing wouldn't move. I ran it for several weeks without being able to use 3rd and 4th gear. The oil sloshing around the tranny and a bit of GENTLE tapping on the shift lever with a hammer freed the rail up without having to dissassemble the tranny.