31x10.50x15's are common, do you have the 16" or 15" wheel? If you want to replace the stock wheels because you don't like the look of them, you could consider painting them. Several people have done that.
For the most part, you can mount whatever size you so desire on the stock rim. I think some people have 33's and 35's on them. Where you would run into problems is if you were trying to run a 31x12.50x15, because there isn't enough sidewall height for the tire to "balloon" to the width.