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 degrees to 350 degrees (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 degrees.
4. 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.