Transfer Case Speed Sensors
WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
There are three speed sensors mounted on the transfer case, 2 on the rear output shaft and one on the front output shaft. Each speed sensor is a permanent magnet (PM) generator. The PM generator produces a AC voltage. The AC voltage level and number of pulses increases as speed increases.