Hi Sarah,
There are some articles on Diane Lee's website which you should read. The one about the stork test and Active Straight Leg Raise is one in particular that will be helpful.
It sounds like your deep hip external rotators are too active. Who knows why. Karen is right when she suggests to stengthen the deep abdominals.
Having said all of that, it does sound like your LEFTSIJ is dysfunctional...it may be causing the right SIJ to anteriorly rotate because the movement is not coming from the left hip, SIJ and L/S. Thus the pelvis is rotating from the right SIJ...
Good luck!