Spray 4-quart slow cooker with cooking spray. Place chicken in slow cooker; sprinkle with 2 1/2 teaspoons salt and 1/4 teaspoon pepper. Pour cooking sauce and chiles over chicken; stir.
Cover; cook on Low heat setting 6 hours or until chicken is tender. Remove chicken from slow cooker; set aside and keep warm. Stir 3 cups of the cheese into sauce in slow cooker until melted; using whisk, stir in sour cream. Replace cover. Use 2 forks to pull chicken into shreds. Place chicken in large bowl. Pour 2 1/2 to 3 cups of the sauce over chicken; toss gently to coat.
Place about 1/2 cup of the chicken mixture on center of each tortilla. Top each with 1 tablespoon of the remaining cheese. Fold one end of tortilla up about 1 inch over filling; fold right and left sides over folded end, overlapping. Fold remaining end down. Top each burrito with 1 to 2 tablespoons of the remaining sauce, if desired. Top with lettuce, sour cream and salsa.