GMC Sierra 2010 Owner's Manual page 497

Manual Shift Transfer Case
A. Fill Plug
B. Drain Plug
Active Transfer Case
A. Fill Plug
B. Drain Plug
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,
located on the transfer case, you will 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.
6-51