Hidden Valley Ranch packets work. You can even make them with FF buttermilk and mayo for less fat. I wouldn't worry about the dairy...it's minimal amounts, so no problem.
You can use a regular ranch, though...any dressing that fits the following is safe on SBD:
- 0, 1, 2, or 3 grams of sugar
- No words with "hydrogenated" in the ingredients
- low amounts of saturated fat
Hope that helps! Enjoy!