all '03+ 4wd's have the larger 12" rotors. Back in the day when those trucks first came out, there was a rash of people having issues fitting 15's. Usually came to being someone trying to run a 10yr old alloy on the truck. Steel wheels never have an issue...alloys are usually always the iffy ones. I've not heard of any 15" wheel fitment issues in a number of years(being part of the ranger community since '03).

I'm willing to bet the shop may not like the 1/16" gap between the caliper and inside of the wheel and wanted to pickup a $150 sale on those stupid ass wheel spacers.

What wheels? What are the wheels specs??? (width, backspace, etc). If you're trying to fit 15x7's, that's possibly the reason for your issues. Get 15x8's. I upgraded to 12" rotors and many many others with 03+ 4x4's run 15's with no issues what so ever. If you wanted to, you could downsize your rotors to the smaller 98-02 size of 11.2". To do this, you'd need the smaller rotors and a new set of steering knuckles.

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'01 XLT disguised as a '03 Fx4 Level II.