GMC 2009 Sierra - Manual (Page 358)

How to Check Lubricant
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.
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.
Rear Axle
When to Check Lubricant
It is not necessary to regularly check rear 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.
All axle assemblies are filled by volume of fluid during
production. They are not filled to reach a certain
level. When checking the fluid level on any axle,
variations in the readings can be caused by factory fill
differences between the minimum and the maximum
fluid volume. Also, if a vehicle has just been driven
before checking the fluid level, it may appear lower
than normal because fluid has traveled out along
the axle tubes and has not drained back to the sump
area. Therefore, a reading taken five minutes after
the vehicle has been driven will appear to have a
lower fluid level than a vehicle that has been stationary
for an hour or two. Remember that the rear axle
assembly must be supported to get a true reading.
AWD Transfer Case (A) Fill Plug (B) Drain Plug
5-46