In this experiment the reason for "surging" to my thinking is this. You excellerate and the TOD senses slipping in the rear by comparing the rear and front driveshaft speeds. The TOD kicks in and moves the vehicle to a point where suddenly the drive shaft rotational speeds match and then the TOD kicks off. When it cuts off there is suddenly a mismatch in speeds so it cuts on. The speeds match again so the TOD cuts off. then on then off then on then off because of continuous mismatch in driveshaft RPM. IT is impossible to get smooth operation with TOD only because with smooth continuous operation the driveshaft speeds agree and the TOD must then kick off.