I have an 02 2wheeldrive polaris 4wheeler. I like to go riding and have decided to upgrade the back tires to allow a little more grip and hopefully be able to tear up some more mud.
So what is a good, rather cheap mud tire for just the back?
And a place in the Houston area to find them, would be great too?

It's not a mud tire per se but it's a great all around tire and you'll never wear it out. Take a look at the Maxxis All Traks. I've run them on several of mine here on the farm and I don't think they can be beat. They're not the cheapest but probably the best bang for your buck and I can't help you on where to find them. Maxxis should have a retailer finder on their website.