Heat oven to 350°F (180°C). Grease a 2-quart baking dish or spray with nonstick cooking spray; set aside.
In a large bowl, stir together chicken, 1 cup of the cheese, condensed soup, milk, sour cream, tomatoes and green chilies, and dry Ranch seasoning.
Place half of the crushed Doritos (about 2 cups) in the bottom of the prepared baking dish.
Top with chicken mixture.
Cover with foil and bake for about 20-25 minutes, or until heated through.
Sprinkle the remaining ½ cup of shredded cheese and remaining crushed Doritos over the top of the casserole. Put the dish back in the oven, uncovered, and bake for about 5 more minutes, or until the cheese is melted.