2008 GMC Sierra 1500 Pickup Owner's Manual - Page 359

How to Check Lubricant For 2WD 6.2L applications, the proper level is from 0.6 inch to 1.6 inches (15 mm to 40 mm) below the bottom of the filler plug hole, located on the rear axle. Add only enough fluid to reach the proper level. What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See Recommended Fluids and Lubricants on page 6-12. Front Axle When to Check and Change Lubricant It is not necessary to regularly check front axle 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. To get an accurate reading, the vehicle should be on a level surface. For AWD 6.2L applications, the proper level is from 0.04 inch to 0.75 inch (1.0 mm to 19.0 mm) below the bottom of the filler plug hole, located on the rear axle. Add only enough fluid to reach the proper level. 5-47