I do not know how to unlock or relax that area, but I do have severe sacroiliac pain. My pain management physician gave me injections in mine and eventually I had RFL or radio frequency lesioning. I did get some relief with that procedure and it lasted about six months. That may help you.
I am receiving weekly spinal adjustment at chiropractor. The lower back area appears more flexible for a day or so but then appears to stiffen up again. Am wondering what exercises might assist; ie Pilates???
Ask your chiro (or research) stretches for the iliopsoas muscles and the hamstrings. That is such an important role in relaxing the SI joints. Just a suggestion from an inactive chiropractor (had to quit practicing because of health issues and chronic pain).
I have the same thing and its very painful. Acupuncture and chiro helps, also I get 12 trigger point inj and an epi every 3-4 months could last longer everyone is different. IT allows me to continue working. Good Luck.