Rear Propshaft Speed Sensor
WARNING: This page is about a different car, the 2005 GMC Yukon XL, 2005 GMC Yukon, 2005 Chevrolet Tahoe, 2005 Chevrolet Suburban, and 2005 Chevrolet Avalanche. 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.