Squeeze juice from limes and orange; set citrus rinds aside. Combine citrus juice, onions, and next 7 ingredients in a blender or food processor; process until well blended.
Remove and discard giblets and neck from chickens. Rinse chickens under cold water, and pat dry. Trim excess fat from chickens. Starting at neck cavities, loosen skin from breasts and drumsticks by gently pushing fingers between skin and meat. Place citrus rinds in body cavities. Lift wing tips up and over backs; tuck under chickens.
Place chickens in a large zip-top plastic bag. Pour juice mixture under loosened skin of chickens. Seal bag, and marinate in refrigerator at least 2 hours, turning bag occasionally.
Soak hickory chunks in water 30 minutes to 1 hour. Drain well.
Prepare charcoal fire in meat smoker; let burn 15 to 20 minutes or until center coals are covered with gray ash. Place soaked hickory chunks on top of coals. Remove chickens from bag, reserving marinade. Place water pan in smoker; add reserved marinade and hot water to pan to within 1 inch of rim.
Place chickens, breast sides up, on rack in smoker, allowing enough room between chickens for air to circulate. Insert meat thermometer into meaty part of thigh, making sure not to touch bone. Cover with lid; cook 3 hours or until thermometer registers 180°. Refill water pan and add charcoal to fire as needed.
Remove chickens from smoker; let stand 10 minutes. Remove and discard skin before eating, if desired (analysis does not include skin).