It will be a gearbox out job unfortunately and you will have to at least split the front off it. If it still selects all 4 forward gears (it will lock the gearbox up if you pop the clutch) but is stuck in reverse then it has slipped out of the gate while coming out of reverse. It may have a worn rod end that may have caused this. Or if it is stuck in reverse and selects no other gear and the gearstick flops around like a cock in a sock then it has jumped out of the gate altogether. But if the gearshift moves nowhere but reverse and it's stuck then there will be something bent somewhere.

The other box's will be no less unknown than the 4 speed .