since you have diffs on a std disco, if you were on soft slippy ground you could lose all forward motion just by 1 wheel slipping, the center diff lock thats engaged by a lever means that to lose forward motion when its engaged you would have to have 2 spinning wheels 1 on each axle, if you put a locker in both axles all 4 wheels would have to spin, because axle lockers are in the axle diff you couldnt easily use a mechanical means of operation so air is often used instead,