Although the pdf is from 2002, it appears as if the following is true (most is the same as previously mentioned, but there are a couple tweaks):
Post 21 (Oak Park): Oakland County exc. Groveland, Holly, and Rose Twps.
Post 24 (Richmond: Macomb and St. Clair Counties
Post 25 (Taylor): Wayne County south and west of Detroit
Post 26 (Ypsilanti): Washtenaw County
Post 27 (Groveland): Groveland, Holly, and Rose Twps in Oakland County (my guess is that MSP is their local PD)
Post 28 (Monroe): Monroe County
Post 29 (Detroit): Detroit, the Grosse Pointes, Harper Woods, and Highland Park