For example, in the aftermath of ACW, in some way the feeling that the two races cannot coexist in the same nation/land becomes official policy, and there is a huge commitment to ship the freed former slaves to someplace in Africa (let's say Congo, or Liberia herself).
At some point pre-ACW, Abraham Lincoln says that he believe the two races are not equal and the best thing the US could do was re-patriate blacks to Africa, so it is not that far off to be considered as US policy. Perhaps a post-ACW US where Lincoln is not assassinated and pushes this polisy?
Yeah, there was much talk about that. The point is, it was really realy expensive. The US may get bankrupt in order to do such a thing on a very large scale, I suppose. However, I think that racism in the country was widespread enough at the time to make the whole thing possible.
Another possiblity is that in subsequent years, racism gets so nasty that American Blacks start organizing themselves to go to Africa, in a way similar to what European Jews did in Palestine. Well, things needed to go REALLY nasty before Zionism became so dominant among the Jews, so I suppose something short of Nazi United States is necessary for an equivalent. And probably it would not qualify for the OP any more than Israel can be described as a German settler colony.