His findagrave.com memorial states he died from a fall when he was home on a furlough from the Army. He had been married just over 2 months when he passed away. Leo was buried on 21 December 1943.
Sources
Find a Grave, database and images (https://www.findagrave.com/memorial/20979590/leo-leonard-jeffs: accessed ), memorial page for Leo Leonard Jeffs (13 Sep 1925–19 Dec 1943), Find a Grave Memorial ID 20979590, citing Parker Memorial Park, Parker, Fremont County, Idaho, USA; Maintained by Kay Stephens (contributor 46910732).
Ancestry.com. 1940 United States Federal Census [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2012. Year: 1940; Census Place: Parker, Fremont, Idaho; Roll: m-t0627-00745; Page: 2A; Enumeration District: 22-24.
Ancestry.com. Idaho, U.S., County Marriage Records, 1864-1967 [database on-line]. Lehi, UT, USA: Ancestry.com Operations, Inc., 2017. Madison County Clerk's Office; Rexburg, Idaho; Madison County Idaho Marriage Records, 1914-1946.
*Ancestry.com. Idaho, U.S., Marriage Records, 1863-1969 [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2005. Upper Snake River Family History Center and Ricks College; Rexburg, Idaho; Idaho Marriages, 1842-1996.