I'm no expert on this topic but I have experience so I'll tell you what I know. I'm running 05-07 20" factory 5-stars on my 04 Excursion. You do need them because from 04 to 05 Ford increased the axle width by 4 inches and therefore reduced rim backspacing by 2" on each side. They also changed from leaf sprung axles to coil sprung. If you don't use them on the front your tires will rub your leaf springs when you turn the wheel hard. Then if you don't use them on the back, your back wheels won't line up with your front wheels which will look weird. I found out the hard way that you want hub-centric spacers. I didn't know there was a difference and I bought some lug-centric spacers off ebay. Had some mild vibrations but some of that was also due to bad tires so I'm not really sure how bad it was due to the spacers. If you don't know, Hub-centric spacers have a raised lip around the inside diameter of the spacer on the wheel mounting surface. The raised lip centers the rim on the spacer/axle because ford lugs are flat and have no wheel locating characteristics as conical lugs do. I'm running steel spacers on the front and aluminum on the back. Not for any specific reason but just because I found 2 used steel ones on ebay and bought the aluminum ones new later. They work just fine and would not hesitate to recommend them. Using spacers to widen the wheel width from factory specs does increase strain on axle components which some will tell you is bad, but that's not what your doing. You are using them to move your wheel width on newer wheels out to where it was with older style rims. Picture an 05 rim with a 2" spacers against the mounting surface and compare that to an 04 rim. The backspacing is now equivalent. Or think of it as a 2 piece rim. Since the backspacing is equivalent, all characteristics are the same. The only negative thought I've considered is of one breaking, but I've never heard of this happening. I hope this helps.
Last edited by regans5; 11-16-2011 at 10:11 AM.