Rear Propshaft 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.
The transfer case shift control module converts the pulsating AC voltage from the rear transfer case speed sensor to a rear propshaft speed in RPM to be used for calculations. The rear propshaft speed can be displayed with a scan tool.