GMC SIERRA 2002 Manual page 400

How to Check Lubricant
Automatic Transfer Case
To get an accurate reading, the vehicle should be on a
Manual Transfer Case
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.
What to Use
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See "Recommended Fluids and
Lubricants" in the Index.
6-33