How do I stop the dog running in front of me and slowing down when out?
 

She crosses to the other side just ahead of me aswell, and when she runs forward in front of me she often slows down, meaning I have to also.

Should the dog always be walking behind when off leash?

This is what German Shepherds do naturally. They are a herding breed.
If you want to control the way your dog walks with you, the only way I see that happening is if you keep her on the leash.

I just don't slow down. My dog learned to get outta my way! ;)

Not unless that's important to you, but you're going to have to spend a lot of time training it if that's what you want. On leash I have rules, off leash we're at the park throwing balls for them and letting them swim, so there are no rules about position, as long as they stay fairly close, they come back to us when called, they stop and wait for us to catch up if we ask them to, and they bring the ball back when we throw it.