First thing, I hope you understand that this plant is not guaranteed to survive one of our winters. I know it is advertised as such, but I am skeptical! If it is stressed during the growing season and does not store enough energy, it is more likely to die during the winter gold. You need to baby these plants until they are well established. Even so, I would protect them over the winter from cold and wind. Off the top of my head, I would make a burlap barrier or a fence around them and pile leaves all around them to insulate them. 

According to Wilson Brothers, this gardenia needs soils with a pH range of  4.5 - 6.0. You are going to be hard pressed to alter the pH to that extent. You need to add elemental sulfur to the soil. Miracid is a fertilizer. It is primarily salts and, if applied at high amounts, will burn the roots of your plants. Instead, adding sulfur will build up in the soil and lower the pH over time. You can buy it at most garden centers--follow the directions on the package for application. 

Right now, the leaves are turning yellow because the plants cannot take up the iron they need because of the high pH. For a quick fix, you can apply chelated iron. Again, this is available at garden centers. Chelated iron will not fix the problem overall--but it will help your plant out temporarily. Long term, you need to lower the pH, but until then, some chelated iron might help. 

Here is a link to a resource from NC State about growing gardenias and includes information on lowering pH and applying chelated iron. Scroll about halfway down the page until you see "yellowing leaves."