Extremely rusty undercarrige
 

What can I do to fix it? It is extremely rusty. It is due to our hellishly salty winters.

Seems like a simple question, but it bothers me.

Clean the rusted parts, cut steel plate to fit in there, and weld it in real nice ;]

Look at the writeup that XJ Stryker did to his Jeeps as he had quite a bit of rust and he repaired it all.