This is actually not insignificant as DNA analyses support a distinctness of the two forms that might lead to them being recognized as separate species one (fine) day.
Having only seen them there and in southern Michigan, I began investigating a bit, although none of the material presented here needed a huge amount of investigation, frankly.
First up, here's a picture (digitalized slide) I took of a Sandhill Crane around the middle of May 2005 on the Upper Peninsular of Michigan:
Wikipedia offers a nice overview of the different forms here.
The most interesting site however was the International Crane Foundation's Sandhill Crane entry which offers a very fine map of the basic migration routes (I can't link there directly, just click it).
Apparently Michigan is only visited by the Greater form, Grus canadensis tabida, both during the breeding and migration season. I am however not entirely sure where the Canadian Sandhill Crane, Grus canadensis rowani, fits in - a form that is apparently intermediate between Greater tabida and Lesser canadensis.
As a matter of fact, the only information I found about rowani is that its subspecific status is in question as it might form some sort of cline between Greater and Lesser.
One of the best places in SE Michigan for watching cranes (alas, I was not to visit it while I was in Ann Arbor) is Phyllis Haehnle Memorial Audubon Sanctuary. According to their "Crane facts", the subspecies occurring there is the Greater Sandhill Crane tabida, so I guess we just ignore rowani for the time being and are happy and satisfied that the Michigan Sandhill Cranes, no matter what the season, are Greater Sandhills.
Which means I really ought to visit the Platte one fine day...