I feel the pain. Being in the northeast, rust is a killer for sure. I had to go to FL to get one without rust.
What are the rest of the details of the transaction though? Before we give "internet advice", we should know the full story. You mentioned that you drove to MS to pick it up, did you pay in full before arriving at the dealership? What was the purchase price? Did you take a test drive before signing anything?
If the body and interior are excellent, but the shock was already detached from the frame from rust when you picked it up, you only paid $1,500, and signed an "as is" document, I would tend to think that the dealer did nothing wrong. On the other hand, if it was misrepresented, you paid $9,000 for it, and sold as "safe to drive and inspection worthy" with a temporary registration tag, then it should be on the dealer as they lied about the condition and you should definitely seek legal advice. But that's just two of the MANY scenarios that could have played out.
But like others have said, some pictures of the undercarriage would help us get an idea on what you're dealing with.
Best of luck with all of this. It's not something that anyone wants to go through.