Chevrolet 2006 Tahoe Owner's Manual page 416

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 shown, 2500 similar
5-48
To get an accurate reading, the vehicle should be on a
level surface.
•
The proper level for the 1500 Series is from
5/8 inch to 1 5/8 inch (15 mm to 40 mm) below the
bottom of the filler plug hole.
•
The proper level for the 2500 Series is from
0 to 1/2 inch (0 to 13 mm) below the bottom of the
filler plug hole.
Add only enough fluid to reach the proper level.
What to Use
To determine what kind of lubricant to use, see
Recommended Fluids and Lubricants on page 6-12.