Rub both sides of each rack of ribs liberally with dry rub. Wrap each rack separately in foil, place on a baking sheet, and cook for 3½ hours or until tender. (Can be prepared to this point up to 2 days ahead. Keep in foil and refrigerate. If preparing ahead, let the ribs sit at room temperature for 30 minutes before proceeding.)
Preheat the broiler to high and position the top rack about 5 inches from the broiler element.
Remove the ribs from the foil and place on a baking sheet. Brush one side of the ribs with barbecue sauce and broil for 5 minutes. Flip the ribs over, brush the other side with barbecue sauce, and broil for another 5 minutes. Serve with the rest of the barbecue sauce on the side.