GMC 2002 Sierra - Manual (Page 126)

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Locking Rear Axle (If Equipped)
If your vehicle has this feature, your locking rear axle
can give you additional traction on snow, mud, ice, sand
or gravel. It works like a standard axle most of the time,
but when one of the rear wheels has no traction and the
other does, this feature will allow the wheel with
traction to move the vehicle.
Four - Wheel Drive (If Equipped)
If your vehicle has four - wheel drive, you can send your
engine’s driving power to all four wheels for extra
traction. To get the most satisfaction out of four
- wheel
drive, you must be familiar with its operation. Read the
part that follows before using four
- wheel drive. See the
appropriate text for the transfer case in your vehicle.
NOTICE:
Driving in the 4 - WHEEL HIGH (4HI) or
4 - WHEEL LOW (4LO) positions for a long time
on dry or wet pavement could shorten the life of
your vehicle’s drivetrain.
Front Axle Locking Feature
The front axle locks and unlocks automatically when
you shift the transfer case. Some delay for the axle to
lock or unlock is normal.