The sacro-iliac joint is a large joint in your lower back in your buttocks region.
You can have injections or manipulation to help with the pain.
An injection mix of local anaesthetic with a steroid helps to confirm the sacro-iliac joint as the real site of your pain, and with time release cortisone this should help reduce the inflammation.
Manipulation can also help decrease pain and increase the range of motion in the joint and is generally done under sedation or general anaesthetic.
I had the latter done and the pain after the manipulation was quite severe but as soon as that died down the sacro-iliac pain has virtually gone. Use of sacro cushions to sit on can also help.