I'm not an expert, but I've read a lot, and here's what I understand:
With stock steel dually wheels, there is no difference. The fronts can be used on the back, and the back can be used on the front. You must turn the wheels inside or outside, depending on which position you want to mount the wheel. But any wheel will fit any position.
But with fancy aluminum wheels, you get four aluminum wheels for the outside positions, plus two steel wheels for the inside dual positions, plus one steel wheel for the spare. Ford finished only one side of the aluminum wheel. So you can still put front or rear on either the rear or front positions, but when you turn them right the ugly side will be facing out.
And you cannot put two aluminum wheels on one hub, so the rear hubs must always have at least one steel wheel. So even though your steel inside rear wheel will fit on the front, you can't put it there because it has to go with one aluminum wheel on the rear hub.
Why did they do that?
Money. Finishing both sides of the aluminum rims would cost more.
I know the lug bore diameters are larger in the rear preventing the fronts from going in the rears but why is that?
I suspect you are misinformed. The front and rear wheels have exactly the same specs. For a '99-'04 models, the front and rear hole pattern is 8 on 170mm, 5.35" nominal offset, on a 16"x6" rim. For '05-up models, the hole pattern is 8 on 200mm, 5.55" nominal offset, on a 17"x6.5" rim.
As somewhat of a proof, your dually came with a steel spare wheel and tire. That spare will fit in any position on your truck, although it may not look great if you replace one of your aluminum rims with it.
So if you have a flat on a tire either front or rear, inside or outside dual, the spare will replace the flat tire until you can get it repaired.