Remove the membrane. Place the ribs so the bone side faces up. Use a butter knife (or your fingers) to lift a corner of the membrane from one end of the rack. Grab the loosened end with a paper towel and pull it off in one slow, steady motion.
Cut the rack and pat them dry. If your rack is too large to fit in your slow cooker, cut it into 2-3 sections to fit. Pat both sides dry with paper towels.
Mix the dry rub. In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, salt, and black pepper.
Coat with Dijon mustard. Use a pastry brush to rub the Dijon mustard over both sides of the ribs (the mustard helps the seasoning stick).
Apply the rub. Generously season both sides of the ribs with the seasoning mix.
Arrange in the slow cooker. Spread ¼ cup of barbecue sauce in a 6-quart (or larger) slow cooker and arrange the ribs to fit. Top with the remaining BBQ sauce.
Cook on low or high. For fall-off-the-bone ribs, cook on LOW for 6-8 hours or HIGH for 4-5 hours (still tender but slightly firm). If possible, baste the ribs with the BBQ sauce every 2-3 hours.
Broil (Optional): Carefully remove the ribs from the slow cooker and place them on a foil-lined baking sheet. Brush with additional BBQ sauce and broil on high for 3 minutes, until caramelized and slightly charred.
Rest and Serve. Let the ribs rest for 5 minutes, then slice between the bones. Serve with extra BBQ sauce if desired.