Step 1: Prepare the Ribs. Begin by removing the thin membrane from the back of the ribs. Use a knife to lift a corner of the membrane, then grip it firmly with a paper towel and pull it off. Pat the ribs dry with paper towels.
Step 2: Apply the Dry Rub. In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, salt, black pepper, and cayenne pepper (if using). Generously sprinkle and massage the rub over all sides of the ribs. For best flavor, let the ribs sit at room temperature for 30 minutes, or refrigerate for 1-2 hours (or overnight).
Step 3: Set up the Slow Cooker. Pour the apple cider vinegar or apple juice into the bottom of your slow cooker. If your rack of ribs is too long, cut it into 2-3 sections to fit. Arrange the ribs vertically around the sides of the slow cooker, or stack them if needed.
Step 4: Cook Slowly. Cover the slow cooker and cook on low for 6-8 hours, or on high for 3-4 hours, until the ribs are incredibly tender and easily pull away from the bone. Cooking time can vary, so check for tenderness towards the end.
Step 5: Finish Under the Broiler. Carefully remove the cooked ribs from the slow cooker and place them on a foil-lined baking sheet. Generously brush both sides of the ribs with your favorite BBQ sauce. Broil for 5-10 minutes, watching closely, until the sauce is bubbly, caramelized, and slightly charred. This creates a delicious sticky glaze.
Step 6: Rest and Serve. Let the ribs rest for a few minutes before slicing them between the bones. Serve hot with your preferred side dishes.