Combine pork, garlic, 1/3 cup cilantro leaves, onion, oregano, orange zest, salt and red pepper flakes in slow cooker bowl and pour broth over top so pork is mostly submerged. Cover and cook on HIGH for 3 hours or LOW for 6 hours or until pork is very tender.
Remove pork from slow cooker using a slotted spoon and let cool slightly. When cool, shred pork and stir together with 1/4 cup cilantro, lime zest and juice, and 1/2 cup cooking liquid. Serve in warmed tortillas with lettuce, sour cream and salsa, if desired.