Interesting that a Nebraska person has not seen a crane since the largest migration roosts are on the Platte River in Nebraska.
Sandhill crane love to roost in the shallow alkaline lakes of the middle Dakotas. I would say between the Missouri River and the James River in SD would be best. We use to pass shoot them coming off these "ponds". If headlights hit the birds even from a mile away they lift off in the dark and are gone. I would suggest you start looking for them between Aberdeen and Mobridge.
Decoying can be hit or miss. Sometimes it is better to be out of the decoys and take them on a pass ... similar to snow geese. Then again, that is the time when a flock bombs right in and lands on the X.
They used to be a bird that would migrate early with peak populations often in late September in ND and maybe a bit later in SD. Now like most geese the cranes also tend to hold in SK as long as possible. So Goosemaster is correct. Saskatchewan.