In a bow, whisk the sour cream and ranch dip together with a little skim milk. You just need enough milk to make it the consistency of dressing. Place in the refrigerator.
Preheat oven to 350 degrees.
Place the tostada shells on a baking sheet. Divide the chicken between the tostadas. Then divide the cheese between the tostadas sprinkling over the chicken. Bake at 350 degrees for 5 to 7 minutes, until the cheese is melted.
Top each tostada with lettuce followed by some green onion and bacon. The drizzle with the ranch. Makes 4 servings.