Company D, St. Clair County Militia is also known as Company D, 60th Enrolled Missouri Militia. Its officers included Captain Benjamin F. Cook, First Lieutenant John Wheeler, and Second Lieutenant Joseph Whitaker. I believe this company was the only EMM unit in St. Clair County.
Perhaps you can contact the local historical and/or genealogical society in the St. Clair County area and see what they can find from local history sources and the older county histories. Osceola is the county seat.