GMC 2011 Sierra Owner's Manual page 349

A Four-Wheel Drive indicator light
comes on when you shift into
four-wheel drive and the front axle
engages. See
.
Some delay between shifting and
when the indicator light comes on is
normal.
Normal
Severe
Extreme
Vehicle in Tow*
*See
.
YES
YES
YES
or
setting also engages the front axle
and delivers extra torque. You may
never need Four-Wheel Drive Low.
It sends maximum power to all
four wheels. You might choose
Four-Wheel Drive Low if you are
driving off-road in deep sand,
deep mud, deep snow, and while
climbing or descending steep hills.
YES
This