GMC 2010 Sierra 1500 Crew Cab Owner's Manual page 426

Use the following procedure to dinghy tow a
four-wheel-drive vehicle:
1. Position the vehicle being towed behind the tow
vehicle and shift the transmission to P (Park).
2. Turn the engine off and firmly set the parking
brake.
3. Securely attach the vehicle being towed to the tow
vehicle.
5-50
Shifting a four-wheel-drive vehicle's transfer case
into N (Neutral) can cause the vehicle to roll even
if the transmission is in P (Park). The driver or
others could be injured. Make sure the parking
brake is firmly set before the transfer case is
shifted to N (Neutral).
4. Shift the transfer case to N (Neutral). See Shifting
into Neutral under
for the proper procedure to select the
Neutral position for the vehicle.
5. Release the parking brake only after the vehicle
being towed is firmly attached to the towing
vehicle.
6. Turn the ignition to LOCK/OFF and remove the
key the steering wheel will still turn.
After towing, see Shifting Out of Neutral under
procedure to take the vehicle out of the Neutral position.
for the proper