Clean the underside of it REALLY well, then spray/paint it with POR-15 or a bedlining product. That should help out immensely against rust.
When I bought my jeep last summer, I thought I only had surface rust (which I have repaired before of different vehicles). This winter I checked it out further as it was gonna be a spring project, and it was worse then I thought. I ended up using my tax refund to have a local bodyman patch weld, prime/paint/clearcoat both my fenders and undercoat them with bedliner. Turned out a close match for the paint and cost me 400$ where as to have two new replacement fenders (shipped to my location) plus the paint job would of been close to 1K.