I had similar situation once, with waste coming out of my basement tenant's shower stall, yet some of the other drains and even the main drain and trap was clear. Had to call a plumber at 11PM, who pulled the basement toilet and snaked (with the big 2" snake) from there. There was a clog along one branch, but not affecting the other branch for the rest of the house. Cost a painful $500 but no way I could have done that on my own without the right tools.