Have you checked out the St. Clair county death indexes on the rootsweb site? There's a Moses Locke who died in 1897, book 3 page 154, but that could be the other Moses Locke who was in the same county. But there's a Sarah D. Lock that's probably yours. She's in the index covering 1868-1880, book 1 page 59.

Here's the link to the rootsweb site:
http://www.rootsweb.com/~mistcla2/

And here's the bio from Andreas' 1883 History of St. Clair County Michigan for the other Moses Locke, maybe the two are related:
Moses LOCKE, farmer, Section 4, P.O. Jeddo, is a native of Canada, and was born August 5, 1823; his boyhood was spent there. After reaching manhood he came to this county in 1845, and he worked at lumbering on Black River until the spring of 1847, then bought land and settled opposite where he now lives. It was all a wilderness, nothing was opened; he built a log house and began clearing his land, making his farm during the summer and lumbering winters, and since then, over one third of century, has resided here; he owns his farm; is engaged in manufacturing molasses, and can also make sugar, and was the fist to engage in the business here; he has held school offices many years; is now Justice of the Peace and member of the Town Board. In 1846, he married Miss D. VANCAMP, a native of Canada. They have one son-George. pg. 752 Grant Twp