The tailgate will lock when you lock the driver’s door with the key, the remote transmitter, the master door lock switch, or the lock tab on the driver’s door.
To unlock the tailgate, turn the key clockwise twice, push the rear of the master door lock switch, or push the UNLOCK button twice on the remote transmitter.

To open the tailgate, push the release, and lift up. To close the tailgate, use the tailgate handle to lower it, then press down on the back edge.

Keep the tailgate closed at all times while driving to avoid damaging the tailgate and to prevent exhaust gas from getting into the interior. See Carbon Monoxide Hazard.
Unlocking the Tailgate
Driver’s Seat Power Adjustments
DTC 71-2x ("x" can be 0 thru 9 or A thru F):
Short in Driver's Seat Position Sensor
Rear Differential Fluid Inspection and
Replacement