1. Whisk together first 6 ingredients in a large bowl until sugar is dissolved; stir in onion and next 2 ingredients.
2. Place buttermilk mixture and chicken in a large zip-top plastic freezer bag; seal and chill 24 hours. Remove chicken from marinade, discarding marinade; pat chicken dry with paper towels.
3. Light 1 side of grill, heating to 300° to 350° (medium) heat. Place chicken, skin side up, over unlit side, and grill, covered with grill lid, 40 to 50 minutes or until a meat thermometer inserted in thickest portion registers 165°. Transfer chicken, skin side down, to lit side of grill, and grill 2 to 3 minutes or until skin is crispy. Let stand 5 minutes before serving.