Try this as this may help - i would be interested if it does.

Have a physio or massage therapist trigger/release (call it what you like) your left TFL/iliacus/psoas - try 1 at a time and reassess your extension based pain and see if 1 or a combination helps relieve your extension pain. Im not sure if this will clear the buttock pain though. Try it and let us know how you went.