2005 GMC Sierra 1500 Pickup Owner's Manual - Page 309

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. Higher Speeds (40 mph (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. Slower Speeds (below 40 mph (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. 4-15