When there were three TV stations, did any of them highlight police brutality? Overincarceration? The military industrial complex?

This is a very fair point. True, having very limited news options didn't allow for a lot of deviation in agreement on observable reality, but to your point, it could also easily paper over a lot of very ugly parts of the actual reality. Chomsky writes quite a lot about this in his book "Manufacturing Consent", which basically is a dive into how media organizations can be used as the propaganda arm of the government. Everything from choosing what you show to choosing how you talk about things goes towards bolstering an underlying narrative that you want to project.

I'm not sure what a solution would look like, if one is even possible. But solution or no, the narrative divergence in this country has primed us to detest each other, which is the first crucial step towards mass violence.