GMC 2003 Sierra 1500 Pickup Owner's Manual page 297

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 Wheel Steer tow mode): When towing a
trailer the 4 wheel steer tow mode provides enhanced
stability allowing the trailer to follow the path of the
tow vehicle more closely, especially during lane
changes.
In this mode the system operates much like the 4 mode,
but is enhanced for trailer towing. It is recommended
for all types and weights of trailers.
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