The (Lutheran) Law/Gospel "dogma"

At Pontifications, Luthodox (Lutheran with Orthodox "tendencies") Chris Jones posed this question to his fellow Lutherans:

Why is the Law/Gospel hermeneutic thought to be the unique way to understand Scripture - the “canonical hermeneutic” if you will? Why are we to believe that this is precisely what St Paul meant by rightly dividing the word of truth? Who among the Fathers before Luther not only used this interpretive method, but gave it the privileged position that the Lutheran Confessions give it?

Don’t get me wrong; it’s a useful and illuminating interpretive principle. But from where I sit it’s no more a fit subject for dogma than (for example) a metaphysical explanation of the Real Presence. I’m open to correction, and I’ve asked before for an explanation of this from my more knowledgeable Lutheran friends. Can you help?

This question prompted this post from Chris Atwood at Here We Stand; if you have any interest in Lutheranism, this post (and the comments) are quite interesting.