By looking at which periods had a lot of members below 50 percent party unity, you can see that, actually, the period between the early 60s and the early 90s (with lots of Boll Weevil/Blue Dogs and Rockefeller Republicans) was something of an anomaly, and things have reverted more to the norm as "Big Sort" dynamics kick in and ticket-splitting dies down. Party unity seemed to take a big hit with the rise of civil rights legislation in the 1960s, with regionalism taking more precedence than party, in the form of many southern Democrats voting against civil rights laws and many northeastern Republicans supporting them.
As you scroll through the years, you can see how that period of overlapping parties started to sort out in the 1970s (after Richard Nixon's "southern strategy"). But it really came to an abrupt end with the 1992 and 1994 elections: redistricting in '92, with many old-school southern Dems being replaced by African-Americans via the creation of new VRA districts, cut a swath, and then the Republican wave of '94 finished the job.
In fact, the polarization of today looks surprisingly like the polarization at previous points in distant history, including much of the late 19th century. For instance, you don't ordinarily think of the 1880s as being a particularly controversial, polarized period in U.S. history (with both parties operating in a fairly conservative framework, arguing over small-bore issues like civil service reform and tariffs) ... and yet, the overall levels and distributions of party unity look strangely like today's. What's different these days, however, is that the red and blue clouds are positioned notably further apart, on the DW/N axis; as you scroll through the years, note how the blue Democratic cloud pretty much always stays in place, but the red GOP cloud steadily drifts to the right during the 1990s and 2000s.
There are a few other neat anomalies worth spotting, like the New Deal years of the 1930s, where the Dem cloud becomes very diffuse instead of a downward slope. That's because even though there was a wide spectrum of DW/N scores within the Democratic Party (largely by virtue of how huge it had gotten at that point), for a few brief years, party unity was still high even among the most conservative members.
(If you want to see corresponding graphs from the 1960s and 1880s, they can be found over the fold.)