Hummer 2004 H2 - Manual (Page 365)

Front Axle
When to Check and Change 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.
How to Check Lubricant
To get an accurate reading, the vehicle should be on a
level surface.
When the differential is cold, the proper level is from
about 1/4 inch (6 mm) to about 3/8 inch (10 mm) below
the filler plug hole.
What to Use
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See Recommended Fluids
and Lubricants on page 6-12.
5-51