2006 GMC Envoy Owner's Manual - Page 403

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. The proper level is from 0 to 3/8 inch (0 to 10 mm) below the bottom of the filler plug hole, located on the rear axle. What to Use To determine what kind of lubricant to use, see Recommended Fluids and Lubricants on page 6-12. How to Check Lubricant To get an accurate reading, the vehicle should be on a level surface. 5-47