On most vehicles, air flowing down the roof establishes a thin, slow-moving "boundary layer" over the surface of the rear windscreen. The boundary layer keeps a certain amount of stuff off the rear windshield, and the high-speed air over the layer carries the rest of it away. Flat-back vehicles like the Explorer, though, don't establish a boundary layer; rather, air whips downward off the roof in a horizontal tornado called a "wake vortex." The wake vortex plasters your vertical rear window with water, mud and snow. With no high-speed airflow going over the window, the only thing standing between you and utter rear blindness is a little swinging arm and a rubber strip.
Pull the wiper arm away from the back of the truck -- it's hinged near the base. Pivot the wiper blade in the arm, and look at the top of the blade, near the pivot and underneath the wiper arm.
Press on the locking tab inside the pivoting joint with your index finger. With the tab depressed, pull the wiper blade downward toward the base of the wiper arm. The pivot pin in the wiper arm will slide off of the hook in the wiper arm.
Slide the new wiper blade back up onto the arm while holding the tab down. This procedure is the same used for the front blades; the only difference between the two is the length of the wiper blade.