Pre-heat the oven to 375 degrees. Spray a baking sheet with cooking spray. Brush both sides of the chicken breasts with bbq sauce and place onto the baking sheet. Top each chicken breast with 2 slices of bacon.
Bake chicken breasts for 25 minutes or until the internal temperature has reached 165 degrees. During the last couple minutes of cooking top each chicken breast with 2 slices of Swiss cheese and allow it to melt. Remove from the oven. While chicken is cooking in a food processor blend together ranch dressing and diced avocado. Set aside.
Lay down one slice of sourdough bread. Top with lettuce and 2 slices of tomato. Top the tomato with a chicken breast. Spread avocado ranch dressing onto the bottom of the top slice of bread. Place on top of the sandwich.