Bruce, thanks for the additions to my understanding of the Laramies.
I've checked the online national register of VA burials, without finding any David Shanks.
It appears that after the war Colonel Shanks moved out west (he's been out west prewar, mining in California).
He was associated, in either friendship or business, with "Col." Allen G. Reed, late of Liberty, MO, who had a freighting business out there and who was later into railroads. In 1868 Shanks was wounded in a gunfight in Laramie in which Col. Reed's son was killed. He is in Laramie in the 1870 census, and in August of that year was elected deputy county constable. An old family document says he died in Denver in/around 1870.
The Reeds are buried in Liberty, BTW.