GMC YUKON XL 2004 Owner's Manual page 317

4: In this mode all four wheels will help steer the vehicle.
If you want to use 4, and your vehicle is not in this mode,
press the button, until the 4 indicator, located to the right
of the 4 Wheel Steer button, comes on and stays on. If
the 4 indicator is flashing you will have to center the
steering wheel by turning it to the left or right.
Slower Speeds (below 40 m.p.h./ 64 km/h )
At slower speeds the front and rear wheels will turn in
opposite directions. This helps the vehicle make
tighter turns, such as during parking, cornering and
turning into tight spaces.
Higher Speeds (40 m.p.h./64 km/h, and above)
At higher speeds the front and rear wheels will turn in
the same direction. This improves stability of the vehicle
during lane changes and sweeping turns.
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