Both of these responses make a lot of sense. I did start off with the full rotation, and have only recently begun making modifications. I had a private session with Terry last year (but was concerned with shoulder problems, not my back) and he showed me the way he now works with arms. That's been fantastic for my stroke and my shoulder.
My video also showed that I was really over-rotating to my right. So I think you're both right--less rotation and building up of core strength. I have done Pilates, yoga, etc., etc., over the years and have seen doctors for my back as well as other joints. But that doesn't mean that I stick with the core strengthening regimen.
I've also started to work more with the side stroke, which actually seems to relieve the pain. My preference would be to work harder on the crawl and get fast, but for now, alternating strokes works best.
Thanks for your help!