How can I reduce my 2" shower drain to match my 1 1/2" ABS drain plumbing?
You should be able to simply use a 2&quot; x 1 1/2&quot; abs slip bushing in the drain outlet, essentially turning that shower drain into an 1 1/2&quot; outlet:

NOTE- the authority having jurisdiction in your area might require a minimum 2&quot; drain size for showers...