I can't find strong sources to back this claim up, although some websites claim that the freed slaves (Americo-Liberians) did subject the local populace to some sort of hierarchical system and possibly forced labor.
If Liberians owned slaves within Liberia they did so without any legal recourse and the strong risk of losing their property.
The claim in the documentary is that the Americo-Liberians, the term for the ruling class of Liberians of American descent, "immediately" went to Liberia and enslaved the natives using something akin to the chattel slavery system to force them to work on plantations. That is absolutely false. The pictures of plantations that can be viewed on the blog might be related the Firestone Company, which operated slave labor in the 1920's, but Liberia was founded in 1842.
The Americo-Liberians believed they were cultural superior and believed they would "civilize" the other Africans. This ended in many wars with the native peoples, considered "civil wars" since they never took the outlook of racial superiority. (wikipedia.org/wiki/History_of_Liberia)
In the 1920's when they joined the League of Nations, the precursor to the modern United Nations, they were investigated for claims that slavery still existed in Liberia by the presidential challenger of Charles D. B. King. It was found: 1. that the native tribes were still using domestic servitude, which could be considered slavery 2. POW's from the civil wars against the native peoples of Liberia and criminals were being used under an unjust "convict labor system" that should be considered slavery "...Vice President Yancy [of Liberia] and other high officials of the Liberian Government, as well as county superintendents and district commissioners, have given their sanction for compulsory recruitment of labor for road construction, for shipment abroad and other work, by the aid and assistance of the Liberian Frontier Force; and have condoned the utilization of this force for purposes of physical compulsion on road construction for the intimidation of villagers, for the humiliation and degradation of chiefs, of captured natives to the coast, there guarding them till the time of shipment [to Fernando Po and Sao Tome.] The Firestone Company was implicated in this scandal.