condition of employment

What is the name of your state (only U.S. law)? WI.
Could I legally be terminated for not being able to get/having a D.O.T. card, if one was not required as a condition when I was hired?
I am diabetic, and we drive bucket trucks and trailers that do not exceed 26,000 lbs. gvwr.

It wasn't a condition of employment then but it is now. Of course it's legal for the terms of your employment to change. If it wasn't, you could never get a raise or a promotion.