This might be blasphemy, recommending a non-d20 game on these boards, but may I recommend a game called Tribe 8 by Dream Pod 9.
Tribe 8 is one of the most detailed and story-driven post apocalyptic games I've ever seen. I won't give away too much of the story, but it deals with the survivors of mankind after a plague of demons arrived in the world. Mankind was saved through the intervention of 8 deities, known as the Fatimas. These seven sisters and one brothers defeated the demons, known as the Z'bri, although the brother, Joshua, gave his life in the process. The Fatimas now rule a tribal theocracy, where the outcasts have few to no rights. You play members of this 8th Tribe (Tribe 8). Its a mature setting (in that it focuses on role-play and themes that do not lend themselves necessarily to combat), designed to appeal to gamers who may not be happy with the type of material major game companies are putting out. (Nothing against White Wolf or WotC or SJ Games, just that Dream Pod 9 has a really great change of pace that I personally enjoy).
The Silhouette System, the game engine that drives the game, is also probably the most elegant and interesting system I have seen. It is a d6 based system where you never roll more than four or five dice (if you are truly amazing at what you are trying to do) and it is known for being a simple system that allows for a lot of rich detail. I know of no other system that allows for this without becoming needlessly complex.
I would highly recommend checking it out, the Canucks at DP9 do truly amazing work.