This started off as a post about the dearth of Post-Apocalyptic Fantasy out there, but more and more examples of it started flooding my brain, so I reconsidered. But then I thought them through, none of them seemed quite... right.
The more I thought about it, the more I realized that it was because while there is quite a bit of Post-Civilization fantasy out there, none of them were Post-Apocalyptic. As I see it, the difference is in the origin of the current state of things.
In Post-Apocalyptic literature, there is a defining event - The Apocalypse, whether that is a meteor striking the earth, zombies rising from the grave, rising water levels, what have you. It doesn't need to be a sudden 90 degree turn that causes the current state of affairs, but looking back, someone needs to be able to say, "Yep, that was when the world went to pot."
Post-Civilization, though is more along the lines of either a) a general decline in the state of things over the course of time, or b) a Post-Apocalyptic world where the Apocalypse happened so long ago that no-one remembers it.
There's lots of examples of Post-Civilization Fantasy out there, but you don't see much Post-Apocalyptic Fantasy out there. Seems like an area worth investigating.