I just went through a bit of a journey trying to identify that track, which included contacting the Ross brothers to ask if they could help me out. What is heard in the video is an unreleased demo of a song from the "Ain't Them Bodies Saints" soundtrack. Unfortunately, the version used in the behind the scenes documentary is not available, but a different, more polished version is available on the soundtrack. The song is "Been Waiting" by Curtis Heath & John Graney. https://youtu.be/krfYWBlXKM0