2004 GMC Envoy Owner's Manual - Page 356

Rear Axle When to Check Lubricant It is not necessary to regularly check rear 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. The proper level is from 0 to 3/8 inch (0 to 10 mm) below the bottom of the filler plug hole. What to Use To determine what kind of lubricant to use, see Recommended Fluids and Lubricants on page 6-13. How to Check Lubricant 5-48