GMC Yukon Owner's Manual page 486

How to Check Lubricant
To get an accurate reading, the vehicle should be on a
level surface.
1500 Series
A: Fill Plug
B: Drain Plug
6-54
All except 1500 Series
A: Fill Plug
B: Drain Plug
When the differential is cold, add enough lubricant
to raise the level from 0 (0 mm) to 1/8 inch
(3.2 mm) below the filler plug hole.
When the differential is at operating temperature
(warm), add enough lubricant to raise the level to
the bottom of the filler plug hole.