Place garlic, onions, orange juice, lime juice and seasoned pork shoulder into a slow cooker. Cover and cook on low heat for 8 hours or high for 4-5 hours.
In a small bowl, combine chili powder, cumin, oregano, salt and pepper. Season pork shoulder with spice mixture, rubbing in thoroughly on all sides.
Remove pork shoulder from the slow cooker and shred the meat before returning to the pot with the juices; season with salt and pepper, to taste, if needed. Cover and keep warm for an additional 30 minutes.