Let me start by saying, so sorry about the sciatica! Last spring, with a little help from my then 6 year old son jumping on my back and pushing me over sideways on my bed, I ruptured a disc in my lower back. It hurt the day I did it, it hurt worse that night, and by the next morning, the pain was god awful. I literally couldn't stand up straight. My right leg was burning, and the foot was pretty much numb.
The Emergency Room doctor, after looking at the x-rays, told me it was going to take WEEKS of bed rest, then I would need to have an operation. Considering the pain I was in, I believed him. He sent me home with about a weeks supply of pain killers, anti inflammatories, muscle relaxants, and anti spas-modals. That was early in the day Saturday. I took pills for about two days, then decreased my dosage for a day and a half before going without them. On Tuesday, I returned to my evening job, still a little uncomfortable, but not bad considering that they told me I wouldn't get out of bed for a few weeks.
That was either April, or May. We went to Disney in August. I drove, 20 hours behind the wheel, from New York to Disney in two days, and did pretty much everything I wanted to. The ONLY ride I skipped out of fear of aggravating the back was Rock N Roller Coaster. I went on Big Thunder Mountain many times. I had the occasional twinge, but nothing too bad, and it didn't last. The only ride that really bothered me was the Kilimanjaro Safari. That bus ride was pretty painful, but again, it was only a problem during the ride, I was fine a few minutes after we got off it.
I guess, my best advice is know your body. Pay attention to what bothers it in day to day life, and you will have a solid idea of what you can tolerate at Disney. Finally, don't be afraid to ask for help. If you are in pain, get a wheel chair and make someone in your family push you around, it is FAR better than being in misery.