In a small bowl, combine margarita mix, lime juice and garlic. Pour 1 cup marinade into a shallow dish. Add chicken; turn to coat. Cover and refrigerate, 2-4 hours. Cover and refrigerate remaining marinade., Drain chicken, discarding marinade. Sprinkle chicken with salt and pepper. Place chicken on oiled grill rack. Grill, covered, over medium heat (or broil 4 in. from heat) until a thermometer reads 165°; 5-7 minutes on each side, basting frequently with reserved marinade.