I checked the IL Death Index Pre 1916, www.cyberdriveillinois.com/departments/archives/databases/de...
but didn't find nancy's death in it.
Deaths were consistently recorded in Illinois until 1916.
You can contact the Johnson County Genealogical & Historical Society, http://johnsoncountyil.net/
and ask if they have 1864 newspapers (for his death notice) and cemetery inventories. They also might have info about his wife too. Successful Searching!