My black lab/shephard mix male dog has suddenly become very aggressive since he's gotten his rabies shot, is this a possible side effects from the shot?

No. How old is he? If this is his first shot at about 6 months it could be just maturing. If he isn't neutered, it would be a good time to do so. The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at Raising Your Dog with the Monks of New Skete For more on being top dog, see Establishing and Keeping Alpha Position, Letting your dog know you are the boss