How about something like a 33% grey to 75% grey gradient left to right under the zip code names which corresponds to the shading of the unselected map regions?

How about something like a 33% grey to 75% grey gradient left to right under the zip code names which corresponds to the shading of the unselected map regions? That way I could see spatial trends on the map that would track your ordering preference. As it is I have to go dousing and try to mentally integrate over time. The 33% still leaves room for the selected zip code to leap out.