Without seeing her, it is hard to say the motivation. If it is fear or guarding you obviously want to get it under control right away.
Regardless, she would be losing the privilege of being off leash when out back so that she doesn't get the opportunity to practice the behavior.
Raven (GSD) - December 8, 2007
Kaiser (GSD) - November 2009
Holly (GSD) - March 24, 2011
Best Paw Forward