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more than likely the lock actuator has failed and will need replaced. give me a few minutes and i will get some more information on repairing this for you.
im having some trouble finding good reference pictures for you, but it is fairly simple to do. you will need to remove the tailgate trim panel. it is held by 2 screws beneath the handle piece, and the rest are just clips. it will pop off with a little force.
once you have the panel off you will see the actuator mechanism connected to your latch piece. simply disconnect 2 reod and the connector. remove the 3 large phillpis scrwes holding the latch in place and it will come out
picture of the connector to the actuator.
found better pic for you. and some more directions that should help
hope this helps. accepts greatly appreciated