I recently rescued a female Jack Russell Terrier/Chihuahua mix that's about 12-15 months old according to the vet. I need help with the only problem I have with her. Off leash, at the dog park, she's fine,runs and plays well with all dogs. On leash, while in the local pet store, when she sees other dogs, she growls and whines. In obedience class, (on leash, of course) she growls, lunges and snaps at the other dogs. When she approaches other dogs, she looks like she wants to get to know them in a friendly manner and then she snaps at them. Other than this issue, she is a great little dog. Any suggestions?