Well, I can say from recent experience, your latch mechanism inside the tailgate might have rusted itself together. I just had this happen on a wagon I've had in storage for sometime.

Eventually, I had to crawl into the cargo area, remove the tailgate trim panel and brutalize the mechanism into submission. If I recall correctly, there are three fasteners securing the latching mechanism. At first I thought, no problem, just undo the fasteners. But, one of them was so welded by rust it could not be undone.

I'd be more specific except it would require me to go out to the wagon and revisit the remnants of the destruction.

Just like your situation, I could unlock the tailgate yet not get the latch to release.

The best I can advise is to carefully pop off the tailgate interior trim panel (a trim panel removal tool is nice to have) and attack the problem head-on.

Richie (near The Burgh)