I've been contemplating this swap since mid way through my burnt burb build. I search for the axles every now and then for the heck of it... Usually $600-$1200 just for the front here.
I still really want to do this on my CC at some point. So I may be asking for a set one day Ethan lol
Start calling your local yards. Here's the listing I referenced earlier. $125 and they have 4 Super Duty trucks in stock today. They can't be the only place, these trucks are lots more common than our rigs.
http://www.grandpajohnsautoparts.com/price-list-2/
GB Craigslist has several ranging from $200-$400 already pulled.