PLACE chicken in shallow dish. Mix next 6 ingredients. Reserve 1/4 cup; pour remaining over chicken. Turn chicken to evenly coat. Refrigerate 2 hours to marinate.
HEAT grill to medium heat. Remove chicken from marinade; discard marinade. Mix barbecue sauce with reserved dressing mixture.
GRILL chicken 30 to 35 min. or until done (165ºF), turning after 15 min. and brushing occasionally with barbecue sauce mixture. Transfer chicken to plate; sprinkle with onions.