Vehicle Speed Sensor
WARNING: This page is about a different car, the 2004 Oldsmobile Bravada and 2004 Buick Rainier. However, it is still accessible from the selected car via links, so may be relevant.
One of the two on the rear output shaft is the vehicle speed sensor (VSS) input to the powertrain control module (PCM). The PCM sends this information to the transfer case shift control module via the Class 2 Serial Data bus.