GMC 2009 Yukon XL Owner's Manual page 450

It is recommended that the vehicle is taken to your
dealer/retailer for service if the headlamps need to be
adjusted. It is possible however, to re-aim the headlamps
as described.
The vehicle should:
•
Be placed so the headlamps are 25 ft. (7.6 m) from
a light colored wall.
•
Have all four tires on a level surface which is level
all the way to the wall.
•
Be placed so it is perpendicular to the wall.
•
Not have any snow, ice, or mud on it.
•
Be fully assembled and all other work stopped while
headlamp aiming is being performed.
•
Normally loaded with a full tank of fuel and one
person or 160 lbs (75 kg) sitting on the driver seat.
•
Have the tires properly inflated.
•
Have the spare tire is in its proper location in the
vehicle.
5-54
Headlamp aiming is done with the vehicle's low-beam
headlamps. The high-beam headlamps will be correctly
aimed if the low-beam headlamps are aimed properly.
To adjust the vertical aim:
1. Open the hood. See Hood Release on page 5-13
for more information.
2. Locate the aim dot on the lens of the low-beam
headlamp.
3. Record the distance from the ground to the aim dot
on the low-beam headlamp.