Does anyone know how to remove the plastic end caps at the ends of the rails on a stock Explorer rack? I need to slide in towers for a Yakima rack.

I've tried to pull them out, but I'm afraid they will break. I've tried to unscrew the last screw holding down the rail, but that is impossible. Any help is appreciated.

The end caps are screwed in. The last screw in the rail is the one that holds the end caps on. that is the screw that has to come off before you can take them off. Try spraying them down with liquid wrench or some other simular product before trying to unscrew them. If that don't do it, then try an impact driver.