Mike, your rear axle is the same one used on the DRW trucks and is rated at 9350 by Dodge and 10,912 by AAM.
Your 17" option is the 285/75/17 Nitto Dura Grappler rated at 3970/tire. My dad has these on his 06 2500 Cummins and Lance 850. With a trailer he is maxed out.
I think you will find that a 245/70R19.5 is your best option. The vision type 81 wheels are rated at 4500lbs and LRG 245/70R19.5's are generally 4540 at 110psi, a perfect match for 9000lbs on the rear.
I personally have around 3000 miles on my LRG 245/70R19.5 setup with Toyo M608z mounted on vision type 81's. I have no intention of going back to LT tires for anything but my 2 weeks of elk hunting. They are smoother, quieter, sturdier, and most of all handle a load like no LT tire can. You do feel the road more, but not in a rough way.
You already have the same frame/axle as a dually and the main leaf pack is the same. All the dually has is 2 more tires and the overloads. The tires are easily fixed with 19.5's and the overloads with airbags, throw a big wig swaybar on there and hit the road.
I don't recommend 265's, with the 0 offset wheels they will likely rub on a stock truck.