Does the F550 have a smallish 10 lug bolt pattern that is only used on the F550? Are you OK using those wheels or do you need larger ones? Are larger ones available?
Rear axles are real easy to swap in this size truck and most any axle you look at and think might fit will fit quite easily.
So find an axle from any application that has the type brakes you want, axle gear ratio, and wheel mounting/bolt pattern to suit. Don't swap an axle in because it's an easy swap but then has undesirable features.
Here are some pictures of an axle swap I did on an F350 a few years ago. Used a Meritor 12 or 15,000 lb axle from an 05 Freightliner medium duty of some description. It was a new take out with suspension and all, just drilled holes in the frame to bolt it in.
It had the large disc brakes I wanted, came with a used pumpkin with 3.73 gears which I wanted, and modern 8 lug hub centered hubs with easy to find 22.5" wheels (matched the front axle I swapped).
https://picasaweb.google.com/lh/photo/WTayjrbZBVsYOtZbQEyGq9MTjNZETYmyPJy0liipFm0?feat=directlinkhttps://picasaweb.google.com/118082...esel6V53T?authuser=0&feat=directlink
