2004 GMC Sierra 1500 Pickup Owner's Manual - Page 459

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 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 1500 HD Series, 2500 Series, 2500 HD, and 3500 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. How to Check Lubricant What to Use To determine what kind of lubricant to use see Recommended Fluids and Lubricants on page 6-14. 1500 Series Shown, 2500 Series Similar 5-59