Place chicken in a large heavy-duty, zip-top plastic bag; add 1/2 cup Citrus Marinade. Seal bag, and marinate in refrigerator 2 hours.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Remove chicken from marinade, discarding marinade. Place chicken on rack; grill, covered, 4 to 5 minutes on each side or until done. Cut chicken into thin slices.
Place 2 cups salad greens on each salad plate. Arrange orange sections, grapefruit sections, and onion rings over salad greens. Top each salad with chicken slices. Drizzle with remaining 1/4 cup plus 2 tablespoons Citrus Marinade.