Transfer Case
WARNING: This page is about a different car, the 2001 GMC Safari, 2001 Chevrolet Astro, 2000 GMC Safari, and 2000 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
Check transfer case fluid level at each engine oil change. Under normal driving conditions, change transmission fluid every 50,000 miles.