How are your body mounts looking especially at the cargo door sill and C pillars? I had one failing and it was binding the latch, didn't know it until almost a year went by. It was just a commuter vehicle for me at that point because the door was such a pain. Eight years of salt and harsh weather, always a battle. It's a good plan to look at the guts, lube em up if nothing else.

 

works 99.9% of the time, otherwise opens and closes smoothly so I doubt it is a mount problem. But it is possible as this poor little guy does have a few rust issues....

 

Will it open while holding the key in the unlock position? If so, probably the wee, circular spring in the latch mechanism is broken.

 

doesn't seem to matter with the key lock. for some reason it always cures itself after a liberal spray of WD40 or penetrating oil and sitting overnight.

 

There is a tiny flat spring inside the rear door locking mechanism which is easy to replace once you get the mechanism out. Used to be a guy on here who’d sell them to you for $0.35. This was one of the more gratifying jobs I once completed. Of course it can also be gratifying to leave the kids in the jump seats when they start acting up as they frantically try to open a rear cargo door that won’t open.