What I was trying to say was if you bought 2 wheels with same rim width, but different offset/backspacing from what you already have wouldn't that have the same effect as using spacers ? (I'm not a fan of spacers). Put one new wheel on each side of the truck, and it moves the rims farther apart.
What I don't know is whether to put the new one on the inside (moving the inside rim toward the diff), or get new rims with the opposite offset (which moves the outside rim toward the fender). Either approach should work by moving the 2 rims further apart, issue would be making sure you have clearance with fender on the outside and clearance with wheel well on the inside while getting enough space between the two wheels.
I think you bought great tires, I was trying to think of the least expensive way to make them work. Gut feeling is replace the 2 inside rims with proper offset - they don't show anyway and using your 2 of your existing wheels outside will match your front wheels. Am pretty sure some duallies come with steel wheels inside and fancier alloy wheels mounted outside.
A more-experienced wheel/tire man could probablly hook you up at the least cost.
