I happen to be a Product Engineer for ECOAT, aka Elctrodeposition, so I will step in here, since my product is what paints all frames and metal parts of most everything you own, including your dishwasher, washing machine, all farming equipment, square D boxes, harley frames, basically anything that metal is exposed to elements of nature. I can assure you the rust you are speaking of does not hurt anything. If you want to cover your entire car with actual paint (final coat) you wouldn’t be able to afford it. Plain and simple. Surface rust is common and not an issue.
When to be concerned is when it’s a thin metal, I.e. muffler, car body etc. For thick surfaces, it’s nothing. Car manufactures use to paint frames, axles etc, but it’ costs money. So much so, for example, that if any manufacture could shave half a mill off their product could save millions of dollars.
The Ford axles are cosmetic issues, nothing more. You should be more concerned of owning a car in Florida and it rusting the paint off the body from the salt in the air, which BTW would rust thru the car body before it even make a dent in the axel.