Remove the silver membrane from the back of the ribs. Cut the ribs so they will fit in your slow cooker. Pat them dry with paper towels and massage the dry rub into the ribs. Evenly distribute the rub over the ribs (make sure you get both sides!).
Nestle the ribs into the slow cooker, and cook for 7-9 hours on LOW, or 4-5 hours on HIGH.
45 minutes before the timer goes off, brush 1/2 cup of barbecue sauce over the ribs. Cover and continue cooking.
Slice and serve with additional barbecue sauce (optional).