Very interesting. But I think I can reconcile the increase of political division and the lessening of other sorts of prejudice with education.
For example, consider racism. As one becomes more educated, it is clear that a wide variety of people from a very wide variety of ethnic background have contributed to knowledge, art, music, and general culture. The same is true for other sorts of prejudice. Factors such as skin color just seem less relevant in a larger context.
But, when it comes to *political* prejudice, the issue is more a question of basic values instead of realizing who has done what. And it is then easier to see history as a grand battle between competing values, which tends to lead to less trust of those on the 'other side'.
And values are much harder to 'educate' into agreement.