Place the peach, mango, garlic, rosemary, thyme, brown sugar, pepper, whiskey, olive oil, and white wine vinegar into a blender. Cover, and puree until smooth.

Pour 2/3 of the marinade into a resealable plastic bag. Add the chicken, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 5 hours. Reserve the remaining marinade in a sealed container in the refrigerator.

Preheat an outdoor grill for medium-high heat, and lightly oil the grate. Place the plank on the grill and lightly brush the plank with olive oil.

Remove the chicken from the marinade, and shake off excess. Discard the remaining marinade. When the plank has started smoking place the chicken on the plank. Cook the chicken breasts until no longer pink in the center and the juices run clear, about 10 minutes per sided. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Brush the chicken with the reserved marinade.