This cache is near the former Launch Facility for a LGM-30F Minuteman II Intercontinental Ballistic Missile. Nicknamed "The Mutt" by its crews, K-9 was manned by elements of the 68th Strategic Missile Squadron, 44th Missile Wing, formerly based at Ellsworth AFB outside Rapid City. Though it originally housed an LGM-30A Minuteman I ICBM in 1963, these were replaced with the Minuteman II between 1971 and 1973. It stood at the ready until the Air Force began retiring the LGM-30F from service starting in December of 1991 and wrapping up in April of 1994, in line with international arms-reduction treaties. The vacant Launch Facilities were left to be imploded and filled in, and are now empty fences surrounding what used to be.
You are looking for a small decon container, hanging on the tree. You do not have to cross the fence or tear anything up to get to it. You will need your own writing stick however
As always, watch for traffic, be safe, good luck and have fun!