I really HATE insurance companies, second-guessing a doctor's recommendation! ARRGGGH. Do you have an appeals process you can go through, with the company? Get your doctor to write a letter, too (and write..don't just call them). Sometimes they'll reverse their "opinion".
And of course get back to your doctor with this "simple" question.. "NOW what??" and see if he has anything he can offer. It is possible to make a diagnosis without an MRI, but they certainly are useful.
I'm really sorry you're in this position. Another thought -- try calling the local chapter of the National MS Society. If there aren't any in your town, they might be in the next larger town. Or go to the National MS Society webpage -- www.nmss.org and look around. They have a list somewhere there of local chapters. They have lawyers that might be able (for free) to help you deal with the insurance company, too.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....