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I would think that the tires would rub together with any size load on the rear.
I've alway have run 235/75R16's and just the way the truck sits, there's not alot of room between the tires, and when it's loaded down, very little room, like maybe an inch between the sidewalls.
As noted, I'm talking about 16" tires, not the 17" that your question is about, but then again, your asking about 265's.
The only other size you can run on the stock rims is 235/80/17 which are about .25" taller. There is a very limited selection of sizes for the dually in 17 inch tires... I have tried to get a taller size and they simply aren't available. Probably the only good news is that there are quite a few brands which are E rated to choose from in the 245/75 and 235/80 sizes and most are reasonably priced between $130-200.
Good luck!