Place the chicken in a large resealable plastic bag; add barbecue sauce. Seal the bag and turn over to coat chicken. Refrigerate at least 2 hours, preferably overnight.
Grill chicken, covered, over medium hot heat for 6-8 minutes on each side or until juices run clear. Or broil for about same amount of time. Cool; cover and refrigerate chicken until chilled. Chop chicken and place in a bowl. Stir in the mayonnaise, onion, celery, salt and pepper flakes. Serve on roll or bun with tomato and lettuce. Enjoy!