It's possible that it might have been an armadillo, though I didn't get the sense that it was alive and moving, whatever it was. It was a big one, if it was, but it certainly made enough clattering beneath me to be something jumping into my undercarriage. I'd still go with the shredded tire theory if pressed, but I wouldn't swear it wasn't an armadillo.

I happen to have written several articles on armadillos of various types, including that one.

http://bookofbeasts.wordpress.com/2012/03/13/nine-banded-armadillo/