First, do you have access to a radar gun so you can establish a baseline and then determine if any adjustments you try actually increase your mph? If not, you'll only be able to make instinctive judgements which may or may not be accurate.
Having said that, I think LeftyRyan is on the right path with his comment about hip and shoulder separation. Basically, you want to delay shoulder rotation as long as possible to give the hips time to fully rotate thereby maximizing hip and shoulder separation. Now you could try to just arbitrarily delay your shoulder rotation. But most of the time pitchers with early shoulder rotation rotate early because of something they do with their posture or glove arm. In your case, I don't see a posture issue. But I do see any area for improvement in the glove arm - specifically in your "opposite & equal".
After your hands break, your throwing arm extends down and back while your glove arm extends straight forward. This means your glove arm takes a shorter path to do its thing than does your throwing arm. And that means your front side is quicker than the back side which leads to early shoulder rotation. Instead of extending your glove arm straight forward, extend it up and forward into a better "opposite & equal" position. Your glove arm will follow a longer path which means it will take longer to do its thing thus delaying shoulder rotation and getting you more hip and shoulder separation. And more MPH.
Oh, and this adjustment will also let you get out further over the front leg because the body is still tracking forward while you're delaying that shoulder rotation.
Andy Pettitte and Tim Lincecum are examples of pitchers whose "opposite & equal" is a low throwing hand and high glove.