GMC 2010 T-Series Owner's Manual page 61

Two-Speed Rear Axle
The Two-Speed rear axle lets the driver select Low
Range and High Range. Low Range provides good
starting torque and pulling power. When selecting High
Range the axle allows for higher road speed and fuel
economy.
For better performance during off-road or under a heavy
load, shift the axle into Low Range and then operate the
transmission normally.
For better performance on the highway, shift the axle to
High Range and operate the transmission normally.
High or Low Range can be used when additional shifts
between transmission gears is needed while driving on
grades and for better fuel economy.
Notice: Driving in Low Range above 50 mph
(80.5 kph) will damage driveline components. Drive
below 50 mph (80.5 kph) in Low Range.
If the vehicle has this feature, there is a label on the
headliner above the windshield or in another place near
the driver that shows how to use it. Be sure to follow the
directions carefully.
Two-Speed Rear Axle Shift Control
Operation
Press the bottom of the switch for High Range and the
top of the switch for Low Range.
Always start the vehicle in motion with the two-speed
axle in low range.
Shifting on a Downgrade
Do not shift the two-speed axle when driving on a
downgrade. The speed of the vehicle moving downhill
may make it hard to shift the axle into either Low or
High Range.
The rear axle shift control
switch for automatic and
manual transmissions is
located in the instrument
panel switchbank.
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