The problem is going to be that either the linkage to the door latch has become unhooked, which is not uncommon. Or the latch itself has failed and has something inside of it, like a piece of metal flashing or something that is not allowing it to unlock. The real big problem you face is that to gain access to any of it the inner door trim pad must be removed. This is going to be real difficult to do without breaking anything, because they are designed to be removed with the door open. I am going to send you the instructions for removing the door trim in hopes that you can work it off of the door without causing too much damage. Then you will be able to inspect the lock for unhooked linkage, etc. Really sorry that there is not an easier way. Disregard any of it that does not apply to your truck, like the widow regulator handle, yours has none. I will ad a second part to the post, just in case it is the rear barn door that you are referring to instead of a rear side door. If it is actually a lift gate or something else, please advise. Thanks
J 38778 Trim Panel Remover
Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.
Remove the trim panel from the door.
Rear Door Trim Panel Replacement (Utility) Removal Procedure