If I'm not mistaken the wheel speed sensors are at each wheel and feedback to the TOD ECU to determine when "slippage" is occuring - the TOD compensates by adjusting more power to front (back is always at "full"). The TOD speed sensors that went out on mine are both on the TOD unit itself - one in front and one behind the transfer case - these measure the speed the internal gears are running and also feedback to the TOD ECU so that it can adjust power to front.

-- John