GMC Denali 2004 Owner's Manual page 233

4: In this mode all four wheels will help steer the
vehicle. If you want to use 4 wheel steering, 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, lights up. 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.
4-13