As said, they are held on by a single bolt from the back the same as the front ones. Bolts often rusted in so lots of penetrating oil if you think you're going to try and use them again. Maybe easier to take the whole bumper off complete with irons so as to get at them easier. Otherwise, attack them carefully with a grinder and then a Dremel maybe. Save the rubber top, they aren't cheap to replace. The rubber acts as a safety buffer when you drop the lower tailgate although it shouldn't actually touch. Might get close if you drop an engine on it though. People also say they are for standing on when loading the roof rack and I guess that works too.
New ones generally available @ $$$$$.