Place the chicken in a shallow baking dish; add barbecue sauce. Turn to coat; cover. Refrigerate overnight., Grill chicken, covered, over medium-high heat until a thermometer reads 165°, 6-8 minutes on each side. Cool; cover and refrigerate chicken until chilled. , Chop chicken; place in a large bowl. Stir in the mayonnaise, onion, celery, salt and pepper flakes. Serve on rolls with tomato and lettuce.