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 (http://www.dogsbestfriend.com/) For more on being top dog, see Establishing and Keeping Alpha Position, Letting your dog know you are the boss (http://www.dogbreedinfo.com./topdogrules.htm)