Dozers move on track and while it moves both tracks move along side. Both tracks can be operated separately and this happens  when the dulldozer wants to turn, at this point one track will move forward and the other backward and then the bulldozer will make a sharp turn. The Dozer track is moved by the transmission. It does not move on front/back wheel or four wheel Drive. Like in cars the four wheel drive is there so as to enable the car move on any terrain. But the Dozer track does same job as the four wheel, it allows the dozer to move and work in any kind of terrain and not only that it balances the weight of the Dozer thereby allowing the dozer remain ontop the ground despite the weight of the dozer.