Vehicles with selectable four-wheel mode can be using
shut down the whole drive train (impulse-shaft,
differencial) in 2WD mode.
Less wear on the drive
train and the front-tires,
easier driving and less
because of the lower roll-resistance.
free-wheel-hubs have the additional advantage, that you
whether they are switched on or not.
Always control both sides and turn on in time!