1. Remove the thin membrane from the back of the ribs.
2. Mix brown sugar, paprika, salt, black pepper, garlic powder, and onion powder in a bowl.
3. Rub the spice mixture all over the ribs.
4. Place the ribs in the slow cooker, standing them up against the sides of the insert.
5. Pour barbecue sauce over the ribs.
6. Cover and cook on low for 8 hours or until the ribs are tender.
7. Preheat the oven to 375°F (190°C).
8. Transfer the ribs to a baking sheet lined with aluminum foil.
9. Brush additional barbecue sauce on the ribs and bake for 10-15 minutes, or until the sauce is caramelized.
10. Let the ribs rest for a few minutes before serving.