Aggresive tendencies

Question
My wife and I are a bit dumbfounded with our dog's behavior lately.  In the company of people he's good, relatively calm dog, and he used to be the same with other dogs.  In fact, he was a joy to take to dog parks because he played so well with other dogs.  However, within the last few months he has become increasingly aggresive with other dogs he meets while on walks.  It's obviously embarrassing to us, but it affects our dog too because we're not able to take him to the dog park any longer, which means he's missing out on good exercise and interaction with other dogs.  He is a two year old Soft Coated Wheaten terrier.  Any thoughts on what we can do to correct his behavior?

Answer
My guess is that if he is a good dog park dog while off leash, that he is not being aggressive on walks, he is being either frustrated, or reactive.  That's really a different issue.  I would advise you to get a copy of "Feisty Fido" by Patricia McConnell, or "Scaredy Dog" by Ali Brown.  Both books have a lot of information about this issue, plus some suggestions on how to deal with it, most of which involve desensitization and attention exercises.  If you can't manage it without help, try contacting someone from IAABC for a consultation.  Good luck.