I came across this thread looking for answers myself. I'm not an expert on this, but I've dealt with it for several years. Here's my story...maybe it will help. I got my lab at 7 months old. A year and a half later, we moved to Mississippi. A couple years after that he started licking his font legs on the top of what I would call, in my case, his forearm. He would lick the skin off and leave an open sore. I went to the vet and they checked for fungus (not the problem), asked about anxiety (not the problem), suggested allergies (which I had tested - not the problem), said maybe it was a combination (could be the problem). I changed from Nutro to D/D and that help. I then changed to Taste of the Wild and it was +/- manageable. At one point I a started giving him a Benedryl tablet twice a day and that helped.
Here's the funny part: About the same time, my eyes started bothering me - red, sore, sensitive to light, couldn't wear contacts. I never resolved my problem with my eyes, but when we moved back to WV in the fall of '10, he quit licking, no more Benedryl, and he was back on Nutro. My eyes felt better than they had in years.
Now, just in the last three months, my eyes hurt and he's licking his legs again. maybe we're allergic to each other and the new house in WV took some time to build up enough of his and my dander to affect each other. I don't know, but it's aggravating.
One more thing, I have a friend that live in central FL. His lab would like her belly clean of fur. He started giving her oils in her food (some sort of dermatological non-Rx oil) and it went away.