1. Mix brown sugar, paprika, salt, black pepper, garlic powder, and onion powder in a bowl.
2. Rub the spice mix all over the ribs.
3. Place the ribs in the slow cooker, meat-side up.
4. Pour barbecue sauce over the ribs.
5. Cover and cook on low for 8 hours.
6. Preheat oven to 375°F (190°C).
7. Transfer ribs to a baking sheet lined with aluminum foil.
8. Brush more barbecue sauce on the ribs.
9. Bake for 15 minutes, until sauce is caramelized.