Recently I left my company of serveral years for a better position. However during my employment I took training that was needed or helped my job which the employor paid for, however they made we sign a paper stating that I would repay the amount if I left within 3 years. When I decided to leave for my new job it had been 2.5 years and now I past employer wants me to repay the full amount and not a prorated amount which I belive is incorrect, but the signed agreement doesn't state one way or the other. In addition, I have been told that is may not be legal / enforceable in Michigan, or that 3 years is longer then allowed, or that they can't force full payment compared to prorated due to not detailing this. Can someone clarify this subject?:)

You better talk to an attorney that deals with contracts. My gut says you owe the money if thats what the agreement said.
I don't know who told you it is not enforceable,but I am sure the contract you signed was drafted by a contract attorney in Michigan. That sounds like wishful thinking that a loophole exists. If you had little bargaining power,it may not be enforceable. It sound like an equal bargaining position to me.
JoeC