One thing that I've noticed as our family goes through the Gospels and Pauline epistles (we're getting around to those of Peter, James, John, and the author of Hebrews of course) is the interesting position of the law,and how when the "Judiazers" oppose Paul (or Peter) on the basis of "the law," they often do not appear to be doing so on the basis of not the written Torah or Pentateuch, but on the "Oral Torah" or commentaries on the books of Moses. Translated, this means that the opposition isn't directly in Genesis, Exodus, Leviticus, Numbers, or Deuteronomy, but rather in traditions of the Pharisees of the day--many of which appear in today's Talmud.
In the same way, we also find that in enforcing this "Oral Torah," many of the provisions of the written Torah are neglected--a famous example being when our Lord chastised the Pharisees for neglecting their own parents if they gave money to the Temple. And so it seems that in the "legalism" Paul and Christ rightly decry, there is an element of lawlessness, or antinomianism.
What about today? Well, there are many who would enforce various extra-Biblical rules on believers--don't drink alcohol, don't use any Bible but the KJV, don't use the KJV, sing only hymns or the metric Psalms, don't sing hymns or the metric Psalms, don't dance, women must wear a skirt or a dress but never pants--but in the application of those extra-Biblical laws, Biblical morality is often neglected. KJV-only advocates viciously attack their opponents, fellow believers are slandered, and accusations are thrown across 17 centuries of history with no substantive evidence.
In short, if you're not living according to the Gospel of Christ, you are likely to become both a legalist and an antinomian, ironically. Given what it takes to become either, this makes sense; whether one begins with works righteousness or a rejection of the moral law of God, one must begin by rejecting the Bible's authority and setting up one's own moral law. Legalism and antinomianism are, in practice, synonymous.