You may have to undo the tailgate handle so that you can get to the rod actuators themselves. Maybe a pull with some vise-grips on each one. Or mainly so you can see what's goin' on in there. Somtimes a rod can fall off or get bent out of shape. Also helps if you push on your tailgate while lifting the handle. Relieves tension.

Now, I had to do this with many ch*vy's. I may not work on your but I can't imagine the design being too far off.

Once open, I would recommend "penetrating grease" not WD-40. (Use that first) The grease sprays like WD-40 but sets up nice and gooey. I love the stuff. You should be able to find it at a good auto parts store. I use the ZEP brand...