: Code requires showers to have a 2" drain and while you might sneak by with the 1-1/2", every drain has to have a trap to block sewer gas from entering the house.
Sorry, I was interupted before I could finish my post. I did not wish to imply that you should try to get by with the 1-1/2" drain for the shower, I was leading up to what alternatives you have. I don't know how far it is to the main drain, but the right way is to break out the concrete to access the 1-1/2" to the drain, replace it with 2" and install a trap. If the drain is not trapped now, you can't use it, so you will have to break out some concrete anyway. I use a small 3 function hammer/drill for small concrete jobs, and it works pretty well. It is what you would want for a major demolition job, but for $100 including bits and chisels, and a little time, you can open that up to gain access. Then fill the trench and finish it with new concrete. Sorry, but there aren't too many options.