suden agression towards visitors

Question
QUESTION: I have a 4 year lab cross that I adopted from a shelter 2 years ago.  One year ago I got a deaf boxer puppy.  They are very hapy together, play all the time and the boxer uses the other ones ears, so to speak as he takes signs from the hearing dog and reacts in the same way, often even faster.  The lab, however, during the past year, has started to become seriously agressive towards anyone who enters our home to the point of baring teeth and snapping.  He barks like crazy and rushes straight at them.  If I lock him up then allow him to enter the room once the guests are in the house he is OK for a while but as soon as they move he acts in the same way as if they were just entering.  He was obviously badly treated before I got him as he is basically quite timid.  I am at my wits end as to know what to do to correct this problem.

ANSWER: It appears to me that your LabX has acquired a fear behavior from your deaf Boxer.  Because this involves a complex set of issues and active human aggression, you need an in person evaluation.  Both dogs need temperament evaluation, apart and together, and need to be observed first hand (by the professional when s/he enters your home), and you need expert advice on how to train/manage a deaf dog, as well as use positive reinforcement training and behavior modification to control your LabX.  Contact the veterinary college in your geographical area and find a certified applied animal behaviorist. Do NOT hire a "dog trainer" who will most likely not have extensive experience in either treating developing front door aggression or the knowledge to manage this "pack" of two who are both working off the handicap of the younger dog.  You need a bona fide expert.  Your LabX is demonstrating fear (and as you report, he is basically "timid".)  Your Boxer is alerting him to signals he would otherwise not obtain.  No one can "fix" this in a text box.

---------- FOLLOW-UP ----------

QUESTION: Well this isn't exactly easy as I live in the British Virgin Islands which is very small and doesn't exactly have a huge population of dog behaviourists.  I would very much like to get somebody like this to visit us here but do not know where to start in trying to find someone.  Please could you recommend somebody or adise me how to find somebody.  The deaf boxer is easy by comparisson to the other one.  He does all the sit, stay, lie, roll over, come stuff like clockwork, in fact he was the best one in the whole dog training class that I took him to.  He is very attached to me and watches my reaction to everything he does or wants to do.

Answer
What does the Boxer do when your LabX behaves aggressively toward visitors?  This might be a clue to the Lab's behavior.  It's almost impossible to diagnose aggression without seeing the dog and/or doing an extensive interview with the owner.  You'll have to provide me with answers:
1.  when did this behavior begin and toward whom
2.  what did the Boxer do when the Lab first demonstrated this
3.  what does the Boxer do now when the Lab shows aggression to visitors
4.  what did you when the Lab first demonstrated this behavior
5.  what do your visitors do when the Lab shows this aggression toward them
6.  Do you put your hands on the dog to control his behavior when he is growling/snarling
7.  what sort of training has the Lab had?  
8.  Has he been thoroughly vetted to rule out physical problems, when was his last complete blood chemistry taken
9.  does he demonstrate any other "odd" behavior you have noticed
10. does he show this fear outdoors on leash and, if so, what do you do
11. any other pertinent information, no matter how small it may seem to you, please add

Please answer these questions in the order I gave them to you.  Be as thorough as possible, no matter how much you have to type.  I can't help you solve this unless I have all the facts and then I can at least try, but your dog may also need some calming medications to help him along.  I don't know how sophisticated your veterinarian is or if s/he is willing to prescribe a beta blocker (propanolol), which will truncate the rush of adrenaline and help the dog's fight/flight mechanism from kicking in unreasonably (as it appears to be doing.)  Also, do some reading and investigation yourself.  Follow this link:
http://www.k9aggression.com/Aggression-Treatment/medical_issues.html

Repost with your answers.  Thanks.