I second the question regarding if your vets are exotics or ferret experts? Both of our ferrets at some point in time had the issue you're describing, our first ferret had developed a funny walk and then began toppling over and was unable to stand on his hind legs. It escalated to him dragging his legs and being unable to use the litter box. It happened out of nowhere, one day he was fine and the next he couldn't walk properly.

We would take our ferrets to an exotics specialty vet, I can't remember exactly what they believed could have been the cause, but we got medicine and what not. No MRI, possibly only an Xray. After some time, he regained the use of his hind legs. I remember looking into getting him a little wheelchair; this situation is so hard, it's heartbreaking to watch them struggle. If their vet isn't a ferret specialist, maybe look in to seeking one, I believe it'll be cheaper than a $4k MRI :(