MIX ranch dressing, Mustard, brown sugar and vinegar in small bowl. Remove 1/2 cup dressing mixture for marinating chicken. Reserve remaining dressing mixture for basting and serving; set aside.
PLACE chicken in large resealable plastic bag or glass dish. Add 1/2 cup dressing mixture. Refrigerate 30 minutes or longer for extra flavor. Remove chicken from marinade. Discard any remaining marinade.
GRILL over medium heat 6 to 7 minutes per side or until cooked through (internal temperature of 165°F), brushing with 2 tablespoons of the reserved dressing mixture during the last few minutes of grilling.
SPREAD remaining dressing mixture on rolls. Top with grilled chicken, lettuce and tomato.