I rushed as a sophomore. There are houses that claim they only take freshman, but I've seen otherwise.
As for dress, first day of open houses, I literally ran to rush from the marching band field, still dressed in my colorguard gear...no one seemed to mind.
Second day of open houses, I wore a jean skirt. Second round, I wore black jeans (new, so they were still jet-black, not grey...) a nice blouse, and a beaded wrap around my waist (good conversation starter - it was really cute). A few girls wore sundresses this round, but mostly it was still "business casual". Prefs, I wore a black skirt and a black and white blouse. Anything more that a sundress or skirt & blouse would be overboard - no cocktail dresses.
As a wise woman here said - Macomb is out in the middle of nowhere, with nothing to do - so you'd think Greek life would be huge. It's not. I wouldn't say that it's looked down on, but Greeks don't rule the school.
Just go through rush (Delta Zeta is ususally good about keeping their website updated with Rush info - I think they've got this year's schedule up).
As for being a sophomore with a 3.0, that should make for a good rush. Recs aren't needed in the least bit, but if you happen to know someone in a sorority, it couldn't hurt (if they could even figure out what to do with it). If you would like to get recs, I can help you locate some chapter alumnae in your area (or give you the name of someone who can). If you "maximize your options", you'll end up somewhere great!
My only other advice, is that I know of at least 3 other people from WIU who post or lurk here often...so save the details of your rush until after it's over. (And then, I'm sure the ladies of GC would love to hear about your rush).
If you have any other questions, feel free to PM me. My chapter is no longer on campus, so obviously, I won't be pressing you toward a particular chapter....