Lmao. First what happens to iron when it isn't painted or protected in some way when exposed to air and moisture? It rusts. No mystery there. So if it really bothers you, you can paint it. It's not a show truck, but you could paint it.
I recently bought a 04 Rumble Bee that is 14 years old. I live in the Midwest and in a humid climate. Since its a collectable truck, I want to preserve it, so I got underneath it. There was some rust on various part and pieces due to the factory never having adequately protected them when new and because of being subjected to salt at times before I bought it.
I bought a quart of Eastwood rust encapsulator (available on their web site) to protect some of the black cast iron parts. Just wire brush or scuff off any scale on the rusty areas and paint on the encapsulator with a brush. It comes in a rattle can too, which would be enough to do those parts you are concerned with on your truck.
You can also scuff the axle housing and shoot it with a rattle can paint too if you want. That's a sub-assembly others can see from ground level. I have used a semi-gloss or matt black finish-whichever turns you on. Might shoot a light coat of primer over the factory original paint before painting the black.
