Place the pork shoulder in the slow cooker. Rub with garlic, salt, cumin, chili powder, black pepper, oregano, cinnamon, and cayenne until evenly coated.
Add lime juice, orange juice, beer, and salsa. Cover and cook on LOW for 8 hours, or until the pork is fork tender.
Shred the meat with two forks directly in the slow cooker.
Preheat the broiler. Line a baking sheet with foil. Spread shredded pork on the sheet and spoon some cooking liquid over the top.
Broil for 5–10 minutes, until the edges of the pork are browned and crispy.
Serve hot on tortillas with cilantro, avocado, salsa, and lime wedges.