GMC Yukon XL Denali 2005 Owner's Manual page 12

Reclining Seatbacks
The vertical power seat control described earlier allows
the seatback to recline.
But do not have a seatback reclined if your vehicle is
moving.
1-6
CAUTION:
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Sitting in a reclined position when your vehicle
is in motion can be dangerous. Even if you
buckle up, your safety belts can not do their
job when you are reclined like this.
The shoulder belt can not do its job. In a
crash, you could go into it, receiving neck or
other injuries.
The lap belt can not do its job either. In a
crash the belt could go up over your abdomen.
The belt forces would be there, not at your
pelvic bones. This could cause serious internal
injuries.
For proper protection when the vehicle is in
motion, have the seatback upright. Then sit
well back in the seat and wear your safety belt
properly.