Cut ribs into serving-sized pieces. Place in a 5-qt. slow cooker. Combine barbecue sauce and salad dressing; pour over ribs. Sprinkle with garlic. Cover and cook on low until meat is tender, 6-7 hours.
Remove meat to a serving platter; keep warm. Skim fat from cooking juices; transfer juices to a small saucepan. Bring liquid to a boil.
Combine flour and water until smooth. Gradually stir into the pan. Bring to a boil; cook and stir until thickened, about 2 minutes. Serve with meat.