And in that case, AutoNation would have the responsibility of correcting that problem before selling you the truck. This is, of course, just our opinion. AutoNation might disagree. In fact, there might even be a warning in your owner's manual that tells you to check the size of your spare before using it. And that's where I'd go if AutoNation elects not to be nice guys and replace your differential and your spare, or at least make a substantial contribution to their replacements.
Differentials are expensive.