Place the pork shoulder in the slow cooker. Roughly chop the garlic and place in the slow cooker (I actually kind of rubbed it onto the meat to get as much garlic flavor as possible, but you could just toss it in, too).
Sprinkle the meat with salt, cumin, chili powder, black pepper, oregano, cinnamon, and cayenne. Rub seasonings onto the pork.
Add lime juice, orange juice, beer, and salsa. Cover and cook on low for 8 hours.
Shred meat with two forks directly in the slow cooker (or take out, shred, and replace in sauce). It should fall apart easily.
Preheat broiler. Line a baking sheet with aluminum foil. Place the shredded meat on the cooking sheet and pour a few spoonfuls of sauce over the top. Broil for 5-10 minutes or until you get browned edges on the pork.
Serve on tortillas with fresh cilantro, avocado, and lime juice.
Recipe by Pinch of Yum at http://pinchofyum.com/easy-crockpot-carnitas