2002 GMC Sonoma Owner's Manual - Page 336

Four-Wheel Drive Lubricant checks in this section also apply to these vehicles. However, there are two additional systems that need lubrication. How to Check Lubricant Transfer Case When to Check Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant. See "Periodic Maintenance Inspections" in the Index. 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'll need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. Use care not to overtighten the plug. 6-26