Maybe you should take it back and see if Disc. Tire will pro-rate the tire and get you a new one. Do they know you air down and actually go off road? If they were worried they couldn't fix it, but decided to anyway, I'd be wondering if they wouldn't be willing to help you get a new one. I rotate my spare in, and I would worry all the time. (but I'm a worry wart anyway) I also don't like the idea of anything happening after you drive on it for a few thousand miles, air up after offroading for the hundreth time, and then have a problem at highway speed. I'm with Tim, take no chances.
One more idea. Contact the dealership (parts dept) and ask them if they have any takeoffs. Sometimes they're willing to make a deal if they've got a lot of them in the back. You're still running the All terrains?