GMC 2003 C4C042 Owner's Manual page 131

Rear Axle Differential Lock Control
(If Equipped)
2-56
If your vehicle has a
controlled traction or
locking differential axle,
the switch is located in the
center of the instrument
panel. You'll see this
control on single rear
axle vehicles.
If you're approaching a slippery surface where it looks
like one or even both wheels may start to slip, you can
press the bottom of this switch. It locks your rear
differential so that power is transmitted equally to both
rear wheels.
Let up on the accelerator before you turn on your rear
axle differential lock.
NOTICE:
-
Activating the rear axle differential lock when
the rear wheels are actually spinning freely, as
they might on ice or snow, can cause you to lose
control of the vehicle. It can also damage the axle.
Turn on this control only when the wheels are not
spinning freely.