GMC Yukon 2012 Owner's Manual page 400

10-32 Vehicle Care
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.
How to Check Lubricant
1500 Series
2500 Series
To get an accurate reading, the
vehicle should be on a level
surface.
For the 1500 series, the proper
level is from 1.0 mm to 19.0 mm
(0.04 in to 0.75 in) below the
bottom of the fill plug hole,
located on the rear axle.
Add only enough fluid to
reach the proper level.
For the 2500 series, the proper
level is from 0 mm to 13 mm (0 in
to 1/2 in) below the bottom of the
fill plug hole, located at the front
of the axle. Add only enough
fluid to reach the proper level.