2008 GMC Yukon Owner's Manual - Page 422

Four-Wheel Drive Lubricant checks in this section also apply to these vehicles. There are two additional systems that need lubrication. How to Check Lubricant Transfer Case When to Check Lubricant It is not necessary to regularly check transfer case fluid unless you suspect there is a leak, or you hear an unusual noise. A fluid loss could indicate a problem. Have it inspected and repaired. Automatic Transfer Case To get an accurate reading, the vehicle should be on a level surface. If the level is below the bottom of the filler plug hole, you will need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. Use care not to overtighten the plug. 5-54