@jcboy,
I've got a girl with environmental allergies who also seems to be a little reactive to corn. Have been very happy with the results I've had feeding Nutrisource Grain-Free Lamb kibble the last few months (switched from a string of Hill's options). Not sure if this is available everywhere, though -- I think it might be a regional product here.
A thought, though. Chewing on the tail base is a classic food allergy thing in a dog, but it can also be a sign of flea allergy dermatitis. Being in Florida and having come from a shelter, I would say the odds of flea exposure before you brought the dog home are pretty high, and the improvement over the past month may be more to do with no flea exposure than to the diet change.
In your shoes, I'd stick with what you're feeding -- I don't know that you're going to get a quality product without chicken, corn, or wheat (usually the big offenders in canine food allergy) for less than that. You could try mixing in a bit of a "normal" diet into a few feedings and see if doggie gets itchy again. If yes, stick with the lamb & rice. If no, the itching probably wasn't due to diet.