2004 Chevrolet Silverado 1500 Pickup Owner's Manual - Page 451

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. How to Check Lubricant 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'll 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. Manual Transfer Case 5-59