Growling from Fear

Question
My husband and I adopted a Boxer/Lab mix at 4 mo. old. She is now almost 10 mo. old. She has always been very attached to the two of us, but very fearful of strangers and other dogs. We took her to training class and socialized publicly regularly and seemed to be doing well, but when we took her to our local dog park for the first time, she growled and snapped at a dog that would not leave her alone and a few days later she growled at two different people at the local pet store. I returned to the dog park and she did great, until another dog kept at her and would not leave her be and she growled at it again. Please help! We love taking our dog with us everywhere and we now worry that she will continue this behavior, or it may get worse.

Answer
I suspect that your pup did not get adequate socialization when she was between the ages of 8-16 weeks, which is the critical socialization period for dogs.  So, she is probably very fearful of those things to which she was not exposed very much during that time.  (Kudos to you for getting her to class, though, as that probably prevented it from being even worse - good for you!) Being on leash often makes it worse, since she may very well feel "trapped".  Nose to nose greetings are not necessarily a good thing to do while dogs are on leash, and I usually avoid them by letting leashes drag on the ground so that I can extricate the dogs if necessary, but also so that no tension is transmitted to them down the lead while the interaction is taking place.
For the un- or under-socialized dog, you may not be able to remediate the situation completely, thus she may be a dog that always needs to be managed around others.  Dog parks are probably not going to be her favorite place - she may always prefer smaller groups of compatible dogs.  However, growling and snapping at an obnoxious dog is NOT abnormal.  It's fine for a dog to tell another dog, in its own language, "buzz off".  In fact, the dog that warns is great - and the dog that snaps, but does not puncture, is great, too.  It could be sooooo much worse.
Of course, you don't want her to hurt a human.  Growling is a warning, and should be heeded.  But, there is much you can do to increase your dog's confidence level so that she will feel less need to be fearful, less need to have to warn, and hopefully, much less reason to think she has to act.  There's a nice booklet called "The Cautious Canine" that can help you condition your dog to accept more and more things, and build her confidence.  Also, another good one is "Help for Your Shy Dog".