In countries like France, Germany and Australia, unemployment was constantly below 4% for many decades after WWII, even below 1% in many cases. What's more, rises in the unemployment rate due to recessions were quickly and successfully fought using fiscal policy. In fact, such policies where quite consciously implemented after WWII even in countries that did not see large scale destruction, because the consensus was that returning soldiers needed to be given employment to avoid social problems.

Unfortunately, low unemployment meant better bargaining positions for workers, and so getting rid of this "problem" was quite openly discussed for some time (this was documented in a PhD thesis, http://bilbo.economicoutlook.net/blog/?p=11941). Economic troubles in the 1970s helped convince people that something needed to change and made it easier for attitudes towards economic policy to be changed, i.e. the shock doctrine. Thatcher in the UK was probably the most memorable driver of this change with her very open fight against unions.

It seems to me that a nice test of which of the two explanations fits the bill better is to compare the development to countries in which those measures of inequality, unemployment, and decoupling of real wage from productivity increases did not change at the same time as in the US.

Was it because the large corporation developed differently, or was it because those political shifts happened at a different time?