If it is milkly like chocolate milk then you have water in the trans. Normally "Strawberry Milkshake" is the slang for finding water in the trans but that is with red fluid. If the trans fluid was dark colored from being old or burnt then it will look like chocolate. I would unquestionable replace your radiator or use an external cooler by itself if you dont want to replace the radiator and completely avoid using your exising in-radiator trans cooler.