What are you doing with the seats and all? I would go to your local auto parts store and buy a ton of undercoating. Comes in spray cans. I would cover a bunch of parts not including suspension, any wiring or exhaust.
91 Grand Wagoneer(Winter vehicle/project)
Prior 08 Wrangler JK Lifted-Miss her
As soon as its off ill offer it for sale.
Carpet and all shaggy trim as well as tan leather seats are looking like new. Door panels ill need to reupholster. Ill also have all woodie hard panels and some brakes suspension parts that are looking good but I'm lifting it and upgrading to rear disks.
First, don't drive it on the salted roads in the winter. And keep it in a garage, or failing that, under cover.
If you must drive it in the winter, you can apply one of the undercarriage treatments like Fluid Film (DIY), or Waxoyl. The more you wash it, the better. Treat any body cavities like inside the doors with cavity wax - available from 3M. I think you'll have to be resigned to it rusting after a few years, if you drive it regularly on the salted roads.
Maybe you'll want to buy a beater to drive in the winter?
I had asked about the spray can stuff from the auto parts store on the bodywork forum and everyone said that stuff sucked and to stay away from it. They said once it cracks it'll start trapping water and make the rust even worse.. They recommended several other options tho. You should check that forum out