You're running into the same problem others are running into. As far as we know, there is no back-to-stock recovery out there for the US Cellular version of the phone. However, I did speak with someone else within the last week who also has an A2 through them, and he said he was able to go back to the store and they can flash the stock ROM back onto the phone.
That said, if you simply rooted but did not remove any bloatware, then all you would need to do to "return to stock" would be removing Superuser, busybox, and su, and you would then be back to where you were prior to rooting. If you have removed bloatware or done any other customizing that can't be easily restored, then I would suggest going to a store to see if they can flash it back to stock as the other person was able to do.