2003 GMC Yukon Owner's Manual - Page 408

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. How to Check Lubricant What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See Part D: Recommended Fluids and Lubricants on page 6-17. Rear Axle When to Check Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant. See Part A: Scheduled Maintenance Services on page 6-4. 1500 Series shown, 2500 similar To get an accurate reading, the vehicle should be on a level surface. The proper level for the 1500 Series is from 5/8 inch to 1 5/8 inch (15 mm to 40 mm) below the bottom of the filler plug hole. The proper level for the 2500 Series is from 0 to 1/2 inch (0 to 13 mm) below the bottom of the filler plug hole. The proper level for the 1500 and 2500 series vehicles with QUADRASTEER™ (4 Wheel Steer) is from 0 to 1/4 inch (0 to 6 mm) below the filler plug hole. Add only enough fluid to reach the proper level. 5-50