I once had a dream that I walked outside and my truck was sitting on cinder blocks and had stolen all 4 of my tires. It was funny cause I woke up and was really pissed off for a couple minutes before I realized it was a dream, hehe.
I'm surprised no one has ever stolen my spare, I had it tied onto the top with nothing more than a rachet tie down for about 2 months and no one touched it. I guess they don't feel like going through all the trouble of getting a 33 off the top of the roof.
If you were that into preventing the theft, you could always have a rear tire carrier built and lock it up. Course that'd be pretty expensive, but hey, it'd look cool