Vehicle Speed Sensor. It picks up a signal from a tone ring on the ring gear. Also used by ABS system.
Are you sure there are sensors on each wheel (I haven't physically seen them on a 6.0L)? The wiring diagram for 03 just shows the differential sensor and the front wheels. It doesn't differentiate between the 7.3L and 6.0L engines.The 6.0L also has ABS sensors at each wheel. The rear differential provides vehicle speed, and does offer some redundancy in the ABS management system (as stated above). That said, primary ABS function though is from the ABS sensors at the front wheels.
ABS front wheel sensor: 03-04, 4WD (left and right): F81Z-2C204-BB
ABS front wheel sensor: 03-04, 2WD, Left: F81Z-2C205-AD
ABS front wheel sensor: 03-04, 2WD, Right: F81Z-2C204-AD
ABS front wheel sensor: 05-07, 2WD (left and right): 5C3Z-2C204-AB
ABS front wheel sensor: 05-07, 4WD (left and right): AC3Z-2C204-A
Ford does call it 4 wheel ABS, but there are only 3 sensors.ABS sensors at each wheel.