I find myself having the same issues. Few things I've done (and continue to do on a daily basis) to prevent the SI issue: I use a lacrosse ball on my glutes rather than a foam roller. This allows a more focused shear as I roll side to side while sitting on the ball (yea, I can hear the comments now...LOL). I then use the lacrosse ball on my lower back.
I place the ball near the spine while laying on my back. I then roll slowly back and forth (I don't cross the spine with the ball. Just do one side at a time). I then lay on my side and place the lacrosse ball under my hips. I usually don't have a problem finding a tight spot in my hips. I then flex and relax while laying on the lacrosse ball. If you want some videos of what this looks like, go to YouTube and search for Kelly Starret.
I'm not a huge CrossFit fan, however, I like what he brings to the table to teach folks how to correct their own issues.
Just my $.02....