My understanding is the Groundhogs, actually hibernate,at least that's what I was told by a Ohio game warden one time. What exactly it takes to put them into hibernation I'm not sure, I imagine it is the temperature.
I know that when I lived in NW Ohio, I seen one on Christmas day one year, it was however a very warm winter and was probably in the 40's that day.
So, that all being said, I doubt your going to see them much more this year, especially in Mi.
You could probably try some bait, maybe carrots or something in the front of the holes. If you just want to get rid of them you could try traps.
Or you could always try some smoke bombs down the holes, not sure how the fits with the laws or your ethics.
But I'm betting your waiting till spring.