Centre Differential

1992 JDM model. I am getting a problem when the steering is on full lock. It's almost as if I had put the fuse in to lock the centre differential. But there is no fuse, so I think the centre differential is locking up. There are no symptoms at all when driving normally. Any other potential suggestions?
If that is the case, then is this a separate physical unit from the gearbox? Can I replace it without having to change the gearbox as well, as the gearbox appears to be OK.
If yes, has anyone got a spare?