Ball aggression is a new one on me.
Dog Aggression, Human Aggression, Resource Guarding. Prey drive, Play drive, Food drive, pack drive.
Usually people talk about ball drive as a positive thing as it can be used to train, and can be indicative on how much persistance a dog will work.
I am not sure if that is what you are dealing with, but if it is, it doesn't have to be fixed, you have to be reconsiled to the fact that you have a German Shepherd Dog.
If you go to the park, leave the balls at home. Lots of dog parks have rules -- no toys, as they can start fights. And GSDs are not necessarily all dog park dogs.
Again, this doesn't so much have to be fixed as it can be used, and worked with and managed.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC