In small bowl, whisk together, tangelo juice, mustard and vinegar; salt and pepper to taste.
Place chicken and marinade in a 1-gallon recloseable food storage bag; seal bag. Toss to coat chicken. Refrigerate 30 minutes or up to 2 hours.
Prepare grill to medium heat. Remove chicken from marinade; discard marinade. Place chicken on grill; grill, turning occasionally, until juices run clear and internal temperature reaches 170°F on a food thermometer (about 15-20 minutes).