[68 TT]

One of my trucks does this now and again even with the door unlocked and having functioned properly the last time I opened & closed it just minutes before let alone years like you are dealing with.

In my case I think it is a worn spot on the latch mechanism gear that is allowing the door to sag just enough that the latch mechanism housing on the door shell is hanging up on the latch portion attached to the cab. It is possible you are dealing with something like this as well. Try gently lifting the door at the back bottom edge right at the rocker panel with a wooden wedge. It only needs to move a very small amount in my truck to get the door to open again. I would guess it is under an 1/8". Worth a shot.

Same truck has a door lock that sometimes when you pull the lock rod up it does not unlock the door. You have to push it back down and pull back up again until it comes up further. You can hear it make a different sound that cues you in to it actually being unlocked. Does the same with the key from the outside. Another possibility for you.

On my 67 Suburban that sat for 34 years in a farmers back yard before I bought it the barn doors were stuck shut. It was the lock / latch mechanism inside the door that was gummed up. Even with the key turned back and forth a few dozen times and moving full range through lock and unlock it wouldn't unlock the latch mechanism. Since the doors were locked the outside button is non-functional so no amount of trying from the outside would ever get it open. Had to pull the access panel off the inside and hose it down with penetrating oil then manually move the parts from the inside to free things up so it would open. This could be your case too. The outside button on the handle does nothing if the door latch is stuck in the locked position.

Do you have access to the inside of the door yet? I would disconnect the linkage rod from the inside door handle and take the lock cylinder out of the door then try working both the lock and the latch release mechanism back and forth several times. After having just rebuilt several inside door handle bell crank mechanisms and seeing how much carnage and slop there is in them this could be contributing to some of your problem on the driver side since it gets the most use and wear. A badly worn inside handle mechanism might not be pulling the linkage far enough to disengage the latch.

The inside door handle does not automatically unlock the door on the 67-71 trucks I have owned unlike some later model ones that will so just trying the inside handle with the door lock mechanism stuck in the locked position is no help.

I am leaning toward your problem being a gummed up lock mechanism inside the door but it could be a combination of some of the mechanical oddities I noted above as well

Hope some of this helps. Good luck.