As mojojojo says, baboons are fully quadrupedal terrestrial monkeys. I.e. baboons don't have brachiation, so they don't have the same movility in shoulder and torso as we or apes, thus making them very unlikely to develop bipedal walk like we did. Instead, they walk on ground like monkeys walk over branches, with the full hand palm over the ground, not knuckles, and their backbone is paralel to the grpund line while on march instead of inclined. In short words, baboons are monkeys that, when trees disappeared in their home, continued to live there without trees.
In the Americas this did not happened. When trees disappeared from a region, leading to grasslands, monkeys just disappeared from that region as well. So who knows. Maybe SA monkeys were just less lucky than African ones.
But there is a problem of order there. The minute the animal just leaves the tree he's not an instant good ground dweller. Meanwhile, ground sloths are expert ground dwellers in that exact moment. It doesn't matter if monkeys could outcompete sloths if given enough time to evolve some better features, because nobody is giving them time to do so in the first place.
So If I understand your correctly ur saying that New World Babboons would be necessary to first compete with the ground sloths, and then once there out of the niche it would be easier for other New World Monkey's to make the transition to a Bipedial species?