Have you checked Ancestry.com's yearbooks listings to see if you can find her in the class of 1975? (If she was born in 1957, that's the year she should have graduated high school, plus or minus a year in either direction) Also, Michigan puts the mother's SSN on birth certificates?? Mine from Mississippi certainly doesn't have my mother's SSN on it.
I have a friend that's younger than you, Eric, and the adoption agency lied to his birth mother and told her that he died as an infant. This was surprisingly common, ostensibly so that the birth mothers would "emotionally move on" but realistically, probably to stop birth parents from looking for their children. I don't know if your birth mother was told this, but if she was, that could be a reason she's not looking for you.