You have to pull those plastic push-pin pieces to release the fender well liner. They pull partially out and then snap back in when you're done. I've pulled them from my TJ several times without problem. Don't forget the top-center holders.
You may want to soften the liner up a bit with a shop light that has an incandescent bulb, it's pretty stiff otherwise. The heat from the shop light will soften it just enough to make it easier to work with.