At the risk of over-simplifying - has an orthopedic vet looked at her leg? The limp in the back end could've been due to a stifle (knee) or hip injury - and the lameness in the front leg could be an effect of the back leg being lame since dogs carry 60% of their body weight on their front legs, and even more so if they are compensating for not using a rear leg.
Perhaps before the neurologist starts taking biopsies of anything, and everyone prepares for the worst case scenario, you might have an ortho vet at least look at the xrays your normal vet took. The appearance of the limp after being on the drug could be a coincidence.
The prednisone should've taken away any inflammation if that's what was causing the limp, which kind of makes me think of an injury that just happened to be at the same time. But if the limp is due to an injury, it will remain.
Just throwing out an idea.
I certainly hope you can get to the bottom of this quickly!