Naval Jelly will work on that just fine most hardware stores carry it

 

Looks normal for a state with salt on the roads in the winter. It can be a tedious job and time consuming but well worth what these guys said about wire brushing everything under there. We have bad winters here in Ohio and I used to spray mine off at least every other day in the winter. Underneath. That really helps a lot. I never had any surface rust on mine. And then each spring just quickly go over with a wire brush and paint everything underneath. You can use various products from undercoating to just paint. I just used paint. The mechanic down the street used to say he could eat off the bottom of my hummer. If you keep up on it its well worth it.

 

If it's too bad sand blast it I had a buddy do my 1982 f150 4x4 step side chassis. Then it was a 3 step rust-o leum process. He used a black diamond sand I believe. We gathered all the sand left on the cardboard to use again I know it must of been expensive. That was a lifetime ago don't know if they do that anymore? That road salt really scales it up in New England so bad that if you were walking your Ball$ would deflate.......Go PAT'S