Suggest you ask your friends in sororities @ UMich, as they're your best source for answering your questions specific to recruitment there.
The Greek Life office might have statistics for you on what %age of new members are in which class (freshman, sophomore, junior, senior). That's another way to get specific answers. Try
http://uuis.umich.edu/greeks/chapter...%20association
There's no DZ chapter listed on the Panhellenic website for UMich. Perhaps your DZ friends attend school elsewhere?