GMC 2007 Sierra - Manual (Page 529)

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.
When to Change Lubricant
Refer to the Maintenance Schedule to determine
how often to change the lubricant. See
Scheduled Maintenance (Gasoline Engine) on
page 617.
What to Use
Refer to the Maintenance Schedule to determine
what kind of lubricant to use. See Recommended
Fluids and Lubricants on page 629.
Front Axle
When to Check Lubricant
It is not necessary to regularly check front axle
fluid unless you suspect there is a leak or you hear
an unusual noise. A fluid loss could indicate a
problem. Have it inspected and repaired.
Automatic Transfer Case
529