'Ah, but Mike,' I hear you cry, 'the post-apocalyptic world genre is already post-modern by its very definition, surely you've not come here to merely spout tautologies?'
No, no, dear reader, comes my avuncular response, you are quite right in suggesting that the genre is post-modern....if , and this is a big if with bells on it, if your conception of Mordernism is comprised entirely turn-of-the-century-Paris complete with Manet's Olympia and Baudelaire slipping his halo on the way to the whorehouse; or, alternatively, a mishmash pop-culture Jackson Pollock, Reagan-era, T.V-Dinner-special where the Godfather is the Last Great Movie and Good Morning Vietnam is the heigth of satire.
Either way, you've got to let the past go. Modernity is the now, or, at least, the perceived conventions of the now and post-modernity is transgressing those conventions in a fashion that still participates to some degree the original qualities possessed by the convention transgressed. Now, holding these truths to be self-evident*, I suggest that the Post-Apocalypse is a vicious and pervasive Modernism, well due for a shift into the the 'Post-'. Had The Road or Fallout 3 been a contemporary of Mad Max, perhaps then it could be justifiable - but no! The genre needs a working over if it's not to be a tired collection of dusty landscapes and hobo-esque fashion design.
The question I put you you is how to go about this. I challenge the community to deliver a Post-Modern Post-Apocalypse.
*As all good truths should be. Holding a truth to be self-evident, by the way, seems like an awful long manner of saying 'in my opinion.'