Yeah I was going to go for 33s at the most, although even that might be pushing it with my tired stock suspension and bushings.Depends on what you are doing with them. Iceland trucks run huge tires 42"+, a heavy loaded strictly rock crawler might not want to push it past 35's. Know your intended purpose, and really by the time you are thinking 37's or 39's you should be able to make your own determination on what your axles can tolerate.
It's just a shame to see people have to swap in Ford axles