Not only have I heard of Willamette, and considered attending the school, but a relative of someone I know very well is in a sorority there. Recruitment is done in the spring, and it is the type of school where undergraduates come at 18, and leave by 22 (and therefore sorority membership is in that age range). Even though you will be 20 (according to your post) you will still be married and a gradaute student. I am not a member of any of those three groups (Alpha Chi Omega, Delta Gamma, and Pi Beta Phi), so I don't know if they consider graduate students or married women for membership. However this document
http://www.willamette.edu/org/phc/phcrecruitrules.pdf does not mention graduate students to be eligible, just sophomores, juniors and seniors for fall COB, and freshmen only for spring formal recruitment.
You should probably contact the Greek Life advisor for clarification and more information.