Prepare the ribs by removing the thin membrane (silver skin) covering the bone. This prevents the ribs from curling as they cook and helps the pork to get tender.
In a small bowl, combine the rub ingredients. Rub all sides of the ribs with rub mixture.
Place the ribs in a large plastic zip bag and refrigerate overnight.
In a small bowl, combine the barbecue sauce and apple butter; set aside.
Cut the ribs into 2-rib portions to fit inside the slow cooker. Cover and cook on low for 4 to 6 hours (or until ribs are 170℉ and test tender with a knife easily piercing into the meat).
Remove the ribs from the slow cooker to a board, and drain off any liquid from the slow cooker. Wipe the slow cooker dry. Place the ribs back into the slow cooker. Pour about ½ cup of the sauce over the ribs. Cover and cook on high about 30 minutes or until glazed. Serve with remaining sauce (warmed), as desired.