2003 GMC Sonoma Owner's Manual - Page 317

Front Axle When to Check and Change Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant and when to change it. See Part A: Scheduled Maintenance Services on page 6-4. If the level is below the bottom of the filler plug hole, you may need to add some lubricant. When the differential is cold, add enough lubricant to raise the level to 1/2 inch (12 mm) below the filler plug hole. When the differential is at operating temperature (warm), add enough lubricant to raise the level to the bottom of the filler plug hole. 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 6-32. To get an accurate reading, the vehicle should be on a level surface. 5-54