There is no specific option for choosing an aft balcony. Those cabins are in a category just like the balcony cabins along the sides, 8A, 8C, etc. You have to look at the deck plans and get the cabin numbers of the aft balconies and then see if those cabins are available.
Some travel websites are better than Carnival's since they will highlight or have flashing arrows by the available cabins. I usually go to one of them and find my cabin, and then quickly book it with my agent.
Which cruise are you looking at?