How to thin Ranch dressing?
Anything, including a 'neutral' flavor, is going to change the flavor, even if all it does is reduce the 'intensity'. So what you need to find is what is an 'acceptable' change. 

With this in mind I suggest your best 'first choice' would be to warm your dressing slightly. Place the container or mixing bowl into a warm water bath. You are not trying to 'cook' or make it 'hot' just warm enough to thin it out without actually changing the flavor profile.

If that doesn't work for you then you might consider adding something to the mix.


The most likely thing you can do is alter the existing ratio to favor the 'thinner' ingredients. More Buttermilk, less mayo or sour cream.
Your second 'best' option would be to seek an thinning agent that does not distort the existing flavor profile 'too much'. Best candidate would probably be heavy cream or soft butter. 
If neither of those options produces an acceptable result you might look for a different thinner such as an oil (olive, soy, canola) or another dairy (milk, low-fat or whole)