According to The Colorado Marriage Index, 1858-1939

These are the listed Shacklett's that were married during that time frame in CO. I do not locate your Howard in there.

Shacklet, Elsie M. Webster, Carter S. 13 Aug 1930 Denver 116131
Shacklett, Clark H. Love, Daisy G. 17 Jun 1928 Denver 110174
Shacklett, Edward Goss, Eva 12 Dec 1902 Pueblo 6686
Shacklett, Esther A. Hipp, Roy L. 12 Jun 1922 Pueblo 19508
Shacklett, Inez Bauer, G. A. 15 Sep 1901 Montezuma 194
Shacklett, Margaret E. Shank, John H. 01 Jun 1935 Denver 4172
Shacklett, Mildred F. Duse, Josephine 18 Nov 1914 El Paso 8190
Shacklett, Richard E. Bartlebough, Jeneva F. 25 Dec 1938 Adams 2412
Shacklette, John L. Lockard, Bessie 30 May 1903 Denver 31389
Shacklette, Lawrence E. Hall, Betty E. 03 Nov 1923 Denver 96868
Shacklette, Lulu West, Bennett A. 14 Feb 1900 Fremont 1803
Shacklette, Omar C. Sipe, Pearl Zenith 29 Apr 1923 Bent 1596