You can do it either way mate, remove the two together as one lump, or you can remove the transfer diff from it's casing, then unbolt the transfer from the gearbox but leave it attached to its mounting brackets, iirc, or you can drop the engine and gearbox as a complete unit and do it that way, it's whatever is easiest for you depending on space, tools available and whether you have someone there to help you which you will deffo want as the gearbox alone is fairly heavy, with the transfer box on it's obviously heavier, but also removing and refitting is a bit more difficult as the transfer box has to go round the back past the flywheel, not really an easy job on axle stands mate!.