2003 GMC Sonoma Owner's Manual - Page 315

Rear 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'll need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. 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. How to Check Lubricant To get an accurate reading, the vehicle should be on a level surface. 5-52