2004 GMC Yukon Owner's Manual - Page 433

Front Axle When to Check 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. How to Check Lubricant To get an accurate reading, the vehicle should be on a level surface. If the level is below the bottom of the filler plug hole, you may need to add some lubricant. 5-55