A differential gear is equipped with a selectively controllable locking device which is self energizing, i.e. it utilizes the differentiation energy to self-lock on its own accord. The control signal is therefore not needed to lock the locking device but rather to selectively, separately for each of the two possible differentiation directions, control it to not lock itself. This gives the differential gear four different working modes. These are: open regardless of differentiation direction; open in one differentiation direction but self-locking in the other direction; open in the other direction but self-locking in the first one; self locking regardless of differentiation direction. A control unit is supplied with sensor data of the present “driving situation”. The control unit has a steering strategy. With the right steering strategy it can regulate the control signal so as to permit the differential gear to equalize the torque at each output shaft for as long as possible but still to practically eliminate the risk of one wheel spin.