I have a 94 full size Blazer that unfortunately had to sit outdoors during the 2 nor'easters that hit the northeast coast while I was stationed overseas. The snow has rusted all the metal on the iundercarriage as well as my tow kit. I'm afraid to even research what it would cost to get this cleaned up. I need some advice here on what I should do before she totally falls apart.

 

Clean off the loose rust and paint and either prime and paint, bedliner, or undercoat it. All easily done in the driveway and quite inexpensive.

 

Pretty much what he said. A method that has worked well for me on many vehicles....

Pressure wash
Remove the loose rust with whatever you have available
Rustoleom primer for rusty metal
Topcoat with satin black paint, my preference is John Deere brand "Blitz black" because it sticks VERY nice to the primer while it is still soft and is durable.
Light coat of aerosol undercoating.
Shop around for the undercoating because the price is all over the place, and I have found they are pretty much all the same in terms of performance.

You can substitute the paint brand of course, but the Deere paint is worth it if you have a dealer around. I buy it in quarts, thin with lacquer thinner and spray it, but it comes in aerosol.