Transfer Case
WARNING: This page is about a different car, the 2002 GMC Sonoma, 2002 GMC Jimmy, 2002 Chevrolet S10 Pickup, and 2002 Chevrolet Blazer. 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 transfer case fluid every 50,000 miles.