We have a german shepard at work(police k9) that started licking the inside of the car and seats on a daily basis. He was starting to have an allergy to his food and soon began vomiting, eating grass. Food change helped and it has since stopped. I think like previously mentioned licking can be a sign of pain, especially licking other objects such as your dog. My JRT has BAD food allergies which are shown by him licking his own feet.
It can also be a grooming thing. Really weird. Stop putting peanut butter on the other dogs butt.