GMC Sierra Denali 2009 Owner's Manual page 317

Two-Wheel-Drive Vehicles
Dinghy Towing
Notice: If a two-wheel-drive vehicle is towed with all
four wheels on the ground, the transmission could be
damaged. The repairs would not be covered by the
vehicle warranty. Do not tow a two-wheel-drive
vehicle with all four wheels on the ground.
Two-wheel-drive vehicles should not be towed with all
four wheels on the ground. Two-wheel-drive
transmissions have no provisions for internal lubrication
while being towed.
To dolly tow a two-wheel-drive vehicle, the vehicle must
be towed with the rear wheels on the dolly. See "Dolly
Towing (Rear Wheels Off the Ground)" following.
Dolly Towing (Front Wheels Off the
Ground)
Notice: If a two-wheel-drive vehicle is towed with the
rear wheels on the ground, the transmission could be
damaged. The repairs would not be covered by the
vehicle warranty. Never tow the vehicle with the rear
wheels on the ground.
Two-wheel-drive vehicles should not be towed with the
rear wheels on the ground. Two-wheel-drive
transmissions have no provisions for internal lubrication
while being towed.
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