Did they live in Mountain Home in 1984, or were they married in Mountain Home in 1984? Idaho holds birth certificates as confidential for 100 years, Death Certificates for 50 years (so if they married in 1984, you're not getting the record). http://www.healthandwelfare.idaho.gov/?TabId=82
Walter Theodor Wiltfang, born 8/7/1904 in Nebraska, died 11/19/1983 in Mansfield, SD.
Ellen Natalia Nelson, born 6/24/1906 in Sandefjord, Norway, died 7/29/1993 in Mansfield, SD.
Dale Wiltfang, born 2 Mar 1929, died Nov 1984 in Kansas City, Jackson Co, Missouri
--The 1930 Census finds Walter T and Ellen N in Warner, Brown Co, SD. Daryl N was 2 11/12, Dale A, 1 1/12. South Dakota Birth Records protected for 100 years so can't find Dorraine's birth info.
In the future, present the information you have on your persons. Identify known dates of birth, dates of death, places of birth, etc. It prevents a volunteer from having to start from scratch. In this particular search, just focus on the basic facts.
There was a Charles R. Aggen, born 6/17/1928, died 9/13/1999, at Mountain Home. Obtain his obit also. He is buried in the Mountain View Cemetery.
Public Records show Charles R. Aggen living at Daniels Rd RR 1 Box 858, Mtn Home. Also listed at this address was a Dorraine A. Aggen, born 5 July 1931. A Bradley W. Aggen, born 3 Sep 1959 also at that address.
For other/further information:
There is a Random Acts of Genealogical Kindness volunteer in Elmore County. http://www.raogk.org/idaho.htm
Ask them to check the city directories (Polks) for Mountain Home in/around 1984, looking for ANY Aggen. That would establish the full name of the Aggen person, the fact they lived there and they were married. If they were not married before 1984, then have them look for public records for the marriage (Not the confidential ones).
Also try to obtain the obituary for Ellen Wiltfang, it should identify the name and location of the daughter (if still living in 1993). Brother Dale's obit, died 1984, might also have her location. Use the Random Acts volunteers for these also...if there is one in those counties.