Jeffrey;
Just some really real armchair ideas, to hopefully sort out where the problem sits:
Have you disconnected the remote control connection from the engine and tried if gearshifts work when operating it directly on the engine/gearbox? In case that works, at least you now know where to hunt for the problem. If the gearshift does not work with direct operation on the gearbox, I guess the gearbox has to come out...

Arne