You may be a year off on the date of the marriage. The 1900 census says that they had been married 17 years and that their son William was born in July 1884 and next son George in Feb. 1886 (the only two born in SD, per that census). The 1910 census says they had been married 27 years.
Vital records were not required in South Dakota until mid-1905. Thus, you might need to search for church records for the marriage.