My Yorkie In The Past Year Has Started To Lick His Front Legs So Much That You Can Tell The Differences In His Legs. What Could Be Causing It?

Allergies, skin parasites, behavioral, and infection-both fungal or bacterial- can lead to licking and self-trauma.  Your veterinarian should help you determine the cause of licking and treat appropriately.
There is no absolute "cure" for allergies--it is about control of the clinical signs.  Anti-histamines, omega-3 fatty acids, frequent bathing, and control of secondary infections are the first treatment steps for allergies.  With severe allergies steroids, allergy shots, and immune modulating drugs are used.
Since he licks you when you stop him from licking his leg this may be obsessive grooming where the problem is behavioral-anxiety or OCD.  Once medical causes are ruled out behavioral are considered.  To treat behavioral obsessive grooming behavior modification and medications are used in combination.  There are several anti-anxiety medications used today for dogs.  I suggest working together with your veterinarian and a behaviorist/trainer to form a plan.  Initially an Elizabethan Collar may be used to prevent the licking and try to break this learned habit.

When was the last time you took it to the groomers, a lot of times small dogs will lick bite and chew on their feet when their anal glands need to be cleaned and most small dogs are unable to do this on their own but the groomers will do this when they bath them..