Will 17 inch wheels fit on a 2004 FX4 that has 18 inch wheels on it from the factory and if so what back spacing is required?

 

yes, the wheels will fit.

No, the tires can't be swapped.

Back spacing will really matter for what tires your using, what lift you have.

Generally anything from about 4-6 inches of backspacing or so would work. Maybe Wandell will chime in on the BS question.

 

The oem wheels have 5.5 inches of bs. I'd stay close to that (like 5 to 6 inches).