1999 GMC Sierra 1500 Pickup Owner's Manual - Page 319

Rear Axle When to Check and Change Lubricant: Refer to the Maintenance Schedule to determine how often to check the lubricantandwhen to changeit.See"RecommendedFluidsandLubricants" in the Index. How to Check iubricant: L PO592 Ifthe level isbelowthebottom of thefillerplughole, you'll needtoadd to raisethelevel to thebottom of the somelubricant.Addenoughlubricant filler plug hole. What to Use: Refer to theMaintenanceScheduletodeterminewhatkind use. See"RecommendedFluidsandLubricants"intheIndex. of lubricant to 6-41