The distance between dual tires is called the offset. For a 225/75R16 it is 10.2 inches, center to center--so the wheel offset is 5.1 inches. For 235 one needs 10.7 inches so the wheels have to be 5.35 offset. Without sufficient space between the duals they will "kiss" and wear very fast. That's why it is so important to keep the pressure in the rear wheels at the makers specification. I thought for a while I might be able to improve ride with a lower rear pressure now that I will have higher load capacity tires. That turns out to be a big fat NO!
This would not be an issue on an F-450--which has a different "hub" with enough clearance for even a 245/75R16 with the stock rims.
The wheels are coming from Southwest Wheels Co
http://www.southwestwheel.com/store/showproduct.aspx?ProductID=1841&SEName=29578
Here is the thread where someone finally found the solution!
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27002484/srt/pa/pging/1/page/1