Auto Air Conditioning & Heater Repair: AC lines corroded, ac lines, rubbery material

Question
I have a 2000 Ford E-350 Super Duty extended 15 passenger van with dual AC units. There are 4 AC lines running parallel front to back along the length of the frame. 2 (side by side on the outside) are completely blistered, black and not capable of containing fluid. I was told "Road Salt" was the cause, yet the other 2 lines running parallel are in perfect shape, and there is NO salt damage or rust to the body or undercarriage at all. The rear compressor is also in need of replacement. What happened? How can I avoid this from happening again? I don't have $2000 to throw at this every 3 years (bought it from a dealer in 2002). Thanks for your help. Photo available.

Answer
What I would do is once you get the lines replaced, Spray the lines with a spray can of UNDERCOATING that they spray on the underside of the vehicles in the eastern U.S to prevent rusting of the frame. It comes in a spray can and is a black rubbery material.