If you are talking about high and low gears, that is done in the transfer box (default
change [W]). To unlock the centre diff, press [D]. Do the same to unlock. [E] and [C] do
front and rear diffs respectively (if manual locking axle diffs are fitted).

If it is real life, if they are the same ratio, it will affect gearing and speedometer.
Different ratios front and rear, hello a very expensive repair bill!