I thought about responding, but, I have even less expertise than Oak!
I have on-going SI joint issues, as well as various hip/back funkiness. I use the foam roller for tight hamstrings and hips, and a lacrosse ball to work the SI joint when it gets out of alignment. I don't spend a ton of time, as too much, especially with the lacrosse ball can hurt more than help. Since both my PT (MacKenzie trained) and LMT prescribed this, I do it. When my son was racing and had severe IT band issues, the PT prescribed the foam roller and it totally "cured" the issue.