I tried this again yesterday and it felt a lot better. Not sure why. I did get some photos, but my new computer and my camera are not getting along. I hope to figure that out tonight or tomorrow.
I have had problems back squatting with even light weights for quite a long time now when using the high bar position--I get odd twinges in my knees, and a sharp strange pain in my right hamstring. The low bar position seems to totally resolve these issues.