As far as us behaviourists have managed to delve into the canine mind, as long as we have been there, we have never seen a dog behave in a jealous way. Jealousy is the way people interpret the way that dogs behave.
The way your dogs are behaving is all to do with status. There may be 17 tennis balls but the dog that believes it is control, has the status of alpha, will see each and every one of those tennis balls as private property.
The answer to this is to take away all toys, do not leave anything lying around. Dogs only get toys when you are there to supervise, play with them and use the toys as a reward for training.