I had a Jeep Grand Cherokee that required all tires to be within spec, but that's because it was a viscous coupling.
But my Hummer H3, Land Rovers, and Toyotas do NOT require replacing all 4-tires if one is worn down, and they are all Fulltime 4wd/awd.
I'm starting to think the "replace 4 tires at once" deal is an old-wives tale based on mechanics experience with early awd systems.
I'd make sure I'd verify in my owners manual before dropping $1200 on new rubber. Maybe Ford is using the cheaper AWD system (makes sense on a crossover), but I figured by 2000+ they'd have wizened up to a system less suspect to tire wear.