Full year resident of Michigan. Do I need to attach the W2 on the mailed return?
Yes, you need to attach a copy of your W-2 along with your Michigan state return.
You do not need to attach your W2 if you are also filing Schedule W
https://www.michigan.gov/documents/taxes/BOOK_MI_1040_WITH_FORMS_674611_7.pdf
Page 40 (page 2 of the Schedule W) states:
If a Schedule W is not included when required, the processing of your return will be delayed. Do not submit W-2 and/or 1099 forms with your return.
Yes, @StefanoN is correct. You do not need to attach your W-2 forms if Schedule W is completed and attached to your Michigan return.
Line 30 of the MI-104 states: "Include Schedule W (do not submit W-2s)"!