Savage Worlds has several fan-created PA settings, mostly conversions of other settings. Savage Gamma World, Savage Redline (from the d20 version by FFG), a zombie adventure (Zombie Run! which starts off a month after the "Night of the Living Dead" and the world is pretty much PA), and also a Savage Fallout. I'm sure there are more, but those are ones I personnaly own.
I think, however, that if you want more rules crunch and a gritty setting, use d20 Modern with d20 Apocolypse and d20 Darwin's World (2nd ed, by the love of all that's holy). With those books, you can strip out what little science-fantasy there is easily and adjust your timeline to the day after to 500+ years after and have a grand ol' time doing it.