This SHOULD unlock naturally through story progression, once you finish the mission "Footsteps of Giants." It's near the end of the game.
Getting lots of complaints that it's not unlocking, usually due to playing co-op. I'm just letting you know where it popped for me while I was playing solo. Unfortunately, I'm not responsible for Gearbox's shoddy coding or it would be fixed by now.
Edit: don't want to say much more than this because, well, spoilers. But you'll get this by playing through the story.