Probably some left over vehicle from WWIII
Haha. Of course. But is it an actual surplus vehicle they used for filming, or did they custom build it? Or maybe it's just the back end and there is no front.
Tweet Frakes and ask him, maybe he'll remember?
"Hey, Jon, remember that little movie you made a few years ago? Good, there is the rear end of a vehicle in one of the scenes. Do you remember what vehicle that is?"
Eh that might be a bit long for a tweet.