Hi Ricks,
I have the same problem with sitting and especially driving. I'm told it's the SI joint and am currently in PT to strengthen the muscles. I'm sorry I can't help with the sitting issue as I haven't figured it out yet. My PT did tell me to keep my stomach muscles pulled it and try to keep my lower back straight (not rounded like when you slump down). This is my third time with this pain and it's been 3 months onging now. It's awful. I really feel for you. Hope you get relief soon.
Linmarie