Mom2b I too had debilitating back pain in my second tri. I went for physio which helped a bit but it was only a temporary fix! The pain would be back a few hours later. I used to use a hot water bottle against the sore area while I was sitting at work which eased it.
The breakthrough came when I started to wear a Carriwell Support band. I suddenly realised that I was pain free while wearing it and then the pain stopped completely!
Also try arnica massage oil and Ice Man Gel. Opinions differ on the safety of both for use in pregnancy, but some doctors say its fine and helps.
Good luck