I forgot about the brake difference on the FX4 vs the XLT. Went back to the store and got the right size.
XLTs could get the 10" too, or at least my '99 has the 10" drums. I think it aprtially depended on whether it had a tow package and which engine. Tow package trucks got them, and I'm guessing all 4.0L trucks probably got them.
Don't feel bad, I've seen a lot of people confused by that. Ford should have just put 10" drums on all of them. I doubt that the 9" ones saved much money or weight and created more complexity for manufacturing and repair.
That said... that truck has 31 spline axles and a unique backing plate... which the backing plates are unobtainium.
Could it be the same backing plate used on the first gen Explorers? They were 31 spline 8.8s with 10" drums.
Sucks that Ford made so many variations in brake parts over the years. I had a similar issue on my old truck. Wheel came off going down the road and slightly damaged the backing plate before I got it stopped. I can find a backing plate for a Ford 9" but apparently there were a lot of different offsets used over the years. Took a while to source one from the right year truck and to this day (3 years later?) I'm still running the slightly damaged one.