There are a couple of things you can do if the rear doors are getting hard to open, but if the handle has gotten rusty and sticky, it's possible that a part has bent, and you can't really fix that without taking the handle out. Unfortunately, the little studs that hold the handles on can shear very easily if they also rust.
If you take a handle off and the studs shear off, you can carefully redrill them for a 1/4 inch thread, and use a standard short bolt. The stud is threaded into a brass insert, and you can drill it out. But you must be careful not to overheat when drilling, or the plastic around the brass insert will melt and it will spin and ruin the handle.
If you do get the handle off, it's pretty easy to see how to rebend things and get everything moving freely again.
You can also readjust the engagement of the handle button. If you look into a hole in the edge of the door, you'll see a little allen head screw. If you have to push the button in too far, you can loosen that screw and alter the linkage. Don't go too far or it won't latch right. If you run out of adjustment and still have to push it too far in, that's a sign that the mechanism inside is bent.
