these types of questions get asked all the time for various counties, and the answer is always did you check the group on the database? The database has a file tab and in that file tab it lists probably close to a dozen or so, police id's, as well as fd and ems for livingston county. In this case I have enclosed the police tab for our convience.
Easiest way to track it down is to enable Radio ID's and hook up Win500 and watch the log, you can correlate who you are hearing to the ID listed in the log. I'd help out but they seem to be on my towers only a half-hour or so on average per day.
All I have is Fire ID's. I'll try to work up a list of Police for you. You might want to send W8MSP a private message and she if he has them. He should be able to help!
The best way to get these is to run Pro96com. I have had a radio running on Pro96com for 2 years on the Howell tower. I'll have to check the log files when I get home and I can give you the range Livingston County PD's run. I'll look and see if there is a specific range the PD's use or if they are mixed with PD, FD and EMS.