Hi Matt, you're getting overdefined messages because you are creating dimensions to sketch geometry in your drawing and that sketch geometry is already constrained to geometry in your model. It's not a big problem, just change the dimensions from Driving to Driven or have SolidWorks automatically make overdefining dimensions driven. You do that in Options: System Options:
You can also create center-line annotations (Annotations toolbar).
You can dimension to these annotations and they're very easy to create (pick the symmetrical edges and it adds the line to the center). Unfortunately, if a centerline loses its reference, it cann't be repaired.
Take a look at those and post back your status.
I'll bet that John's top screenshot will fix your problems. I wanted to mention that in my case that box occasionally un-checks itself for no apparent reason, so if the behavior you're describing reoccurs you might want to go back and check it.