It sounds like PGP/pelvic instability to me - is it just that you don't think it's SPD, or you don't think it's PGP either? I have found my SRC support shorts a lifesaver - I went from barely able to walk and seriously considering that hubby would have to go on carer's leave as I couldn't cope with the kids, to moving around quite well. I still need to be careful and I'm achey at the end of the day, but it's SO much better than it was.
I'm also doing a physio-run exercise class which I find brilliant. Activates all the core/glute/thigh muscles that hold your pelvis together and it honestly feels like I'm wearing a corset for 24 hours after! Everything's all engaged and stable. Then bit by bit I lose power and go wonky through the week! Wish I could afford to do it three nights a week - pelvis would be appear and it could well save th life of my poor flagging pelvic floor.
Hope you find something that works for you. xo