My 2 year old GSD has started attacking my 9 year old Lab. What can I do?

Dear Sir:

I have a female German Shepard about 2 years old, who has been spayed. She has always been a lovable dog and still is, but in the last 2 weeks she has become aggressive toward my 9 year lab, who is also a female and has been spayed. She never offers to fight with this dog when they are eating, in fact, if the lab goes over to the shepard's pile of food she will move to the other pile of food. The shepard has acts like she is trying to dominate the lab. She will go up to her and put her foot on the lab's back, and her ears go up, and then she wants to jump on the lab, and she has drawn blood with her attack on the dog. The lab is such a humble dog and goes down in a ball and doesn't try to defend herself. I have a chow that I took in a year ago, and the shepard does not do this to the chow, he is a male. The shepard plays with the chow, but doesn't fight with this dog. I have noticed that when the shepard jumps on the lab, the chow sometimes joins in with the attack on the lab, but the rest of the time, the lab and the chow get along fine. I talked to the vet and he thinks it is the shepard wanted to be top dog since she has matured, but I don't want her hurting my lab, she is such a sweet dog, and whenever the shepard attacks her, later she will start licking the shepard's fur. These two dogs will lick each other in the mouth and be friendly, then all of a sudden the shepard wants to be aggressive. I try to show all the dogs affection when I show one affection. I will try to get the lab to walk next door with me and the other dogs, and the shepard will stand and stare at the lab, and the lab will go back home. Can you give me any ideas as to what is going on, and what I should do. I do no want to get rid of any of the dogs, but I don't want my lab hurt.

Thanks,
Linda