You posted in the Sheridan county, Montana, message board. All data points to Sheridan, Wyoming, not in Montana.
I see you are new to genealogy posting, at least under this user name. When you post, it is important to fully identify who you seek, full dates of birth/etc are ALWAYS appreciated. So volunteers don't have to do a lot of initial research to just identify who you are talking about.
If you have a posted family tree:http://trees.ancestry.com/tree/25731665/person/1703212153
identify the link. It saves you a lot of 'splaning' and will give a volunteer nough information to actually find who you are talking about.
The Social Security Death Index:
William T. Wilson, born 8 August 1910, died 16 Dec 1993, "last residence" Sheridan, Sheridan, Wyoming. The "last residence" on the SSDI is NOT necessarily where the person lived. It is NOT based on any benefit. It is known only by the "home of record" to the Social Security Administration. I have seen this go back to the original application made for a SSN. His SSN 503-20-2651, was issued in South Dakota before 1951.
--You also show the SSDI as Source.
--Your tree shows two sons and one daughter. It appears you do not know where they are ? If you do, contact them for information.
One other family tree http://trees.ancestry.com/tree/11924324/person/1032841582?ss...
identifies him as William Theodore, death in Sheridan/Source: SSDI; her, as Mary Helen Miller, born 11 Jan 1914 in Belle Fourche, Butte County, South Dakota, death 22 August 2000 in Sheridan/Source: SSDI.
--Since you have an ancestry.com membership, contact this owner, sometimes they have more information than on their tree.
--Also a private tree:http://trees.ancestry.com/pt/RequestTreeAccess.aspx?tid=1288...
This person also has five other trees with thousands of names. Usually people with so many names are just tree builders, adding whatever information found without full sourcing...Ancestry World tree, etc, unverified and sometimes wrong information.
The Sheridan Press has online obituaries, but only goes back to January 2001.http://www.thesheridanpress.com/newobituary.htm
Your best bet would be a volunteer with the Random Acts of Genealogical Kindness in Sheridan County. Public Records (library) might find them in City Directories. Public Records (Courthouse) would find property ownership, possible Power-of-Attorney's, Probate records, etc.
READ the guidelines first:http://www.raogk.org/wyoming.htm