personally, if it assembled once correctly and seperated, and its not hollandaise or mayonaise...
I'd whip out my stick blender, pour it into a tall container suited for the blender, and just pulse it. it'll get back in line. I'd do this for a typical emulsion type mix such as a vinegrette...something creamy with egg/oil mix won't help.
it might save the ranch, though. have to try to see if it'll work with blending, or if you absolutely need to seperate and re-add the oil as if you were making mayo. A spot on technique though. :) definately a good way to make sure it gets integrated.