Myopic w/ astigmatism & ms, I've had increasing double vision despite being treated w/ tysabri (natalizumab). For correction, do I need ophthalmologist or neuro-op?

Depends . If your double vision goes away when you close either eye, it's a muscle problem and a neuro-oph would be best (though an ophthalmologist could always see you too). If the double vision seems to be only in one eye, then it is a refractive problem and you should see an ophthalmologist. Could be anything from needing glasses to having a cataract.
Double vision. The double vision is more likely to be due to multiple sclerosis than due to an intrinsic eye problem. Perhaps a neuro-ophthalmologist could provide more insight on your problem, prism may be an alternative. Most cases of double vision due to ms resolve over time. Solumedrol (methylprednisolone) may accelerate the recovery. All other treatment for ms (from Copaxone to tysabri) are intended to reduce relapse rate and are not expected to help or improve any present symptoms related to multiple sclerosis.
Complex. You are taking a very potent medication, and I would assume the Solumedrol (methylprednisolone) is being used to treat a relapse. As you start to remit, the double vision may seem to worsen, but in fact, your eye maybe responding and stabilizing. Under the circumstances, a neuro-opthalmologist would be helpful, but many general eye specialists could also assist you. But have a discussion with your neurologist also.