I had a look through the gearbox rebuild guide i have for the mt75 and it says to do exactly what you are doing! special tool to hold the splined input shaft and long reach socket on the nut and undo? when i pulled apart an old mt75 4x4 box i done what you are but used a breaker bar on that 1 i think but you have to properly support the socket with wood or something to save damaging the shaft