I plotted data from the Census Bureau Historical Household Income Tables to get these graphs.
First, here are the upper income limits for the bottom 4 quintiles, along with the lower limit for the 95th percentile for years from 1967 through 2017.
Looking at it in constant 2017 dollars in Graph 2 makes this picture even more stark.
The modest nominal gains in the bottom two quintiles have been largely obliterated by inflation. The spread between groups has widened.
What is the mechanism for increased disparity? The data shows that it is income captured by each group. This is presented in Graph 3.
Graph 4 shows the 1st and 4th quintiles along with the top 5%.
The top 5% have gained a significantly increasing share of the pie, and now are receiving about as much as the entire 4th quintile. The pie is growing, but the rich are taking an increasingly larger slice.
I haven't taken a hard look yet at the article I linked at the beginning of this post. We'll see what kind of arguments are put forth to counter the reality I have presented here.