I think I know exactly what you are talking about. Just this morning I decided to check this forum to find a solution for my severe thigh, hip, knee, sacrum (coccyx) pain. My pain which starts at the hip and down the sides of both legs and knees wakes me up during the night and the only relief I can get is to turn on my back and straighten my legs our in front of me. I have a hard time trying to sleep on my back so every time I unconsciously turn on my side the pain starts again. I did get an MRI which showed L5S1 facet joint arthritis (that’s the sacrum). I’ve scoured the internet and don’t come up with much information. Doctors are not very helpful. They just want to get you in and get you out. So the pain remains and I have no real solutions. Please let me know if you have found any reliable information.