I just had the best ranch dressing in Ekalaka Mt so I decided I need to copy it! You can make it thicker or thinner as desired, but man did I love it. I don't know if it matches their recipe or not, but its my best guess!
Mix all and let set in fridge for at least an hour to blend flavors. If you want it a little thinner, use a little more milk, but watch how much you put inches A little bit goes a long way (at least for me!).