just a thought
I would think mountain biking would be more jarring than road biking. Why not get a touring style road bike and still ride on the road?
Anyway, you should check out the Juliana from Santa cruz and the Stumpjumper womens FSR from Specialized.
I would think a full suspension bike would be the way to go after back surgery. Or maybe a custom built steel/ti frame.
My wife had a disc fusion last year and is back riding her Rockhopper a couple of times a week now. Good luck with your surgery