Social means a lot of things, societal from what I see is more limited in its definition. It seems that societal's arrival might be correlated with the popularity of Marxism since it was created circa 1900. Societal is not redundant since it can be used for accuracy and precision. The problem with using societal is that it is perceived by some to be a register change from social, although it is not. Also, if you are in Sociology class you can throw your English dictionaries out the window for this problem and ask for clarification with your instructor or buy a Sociology dictionary, because in this case it could just be a jargon issue.