The Peacekeepers are all gone now, part of the START II treaty provisions, but the advanced warheads will likely be moved to the existing Minuteman III missiles when they are downsized from five warheads to a single re-entry vehicle.
According to a former SAC missile guy here at the office, the site we're looking at is the training area for the missile technicians. The silo mock-up on the right is a Minuteman III site and the one on the left, with the white trucks, is a Peacekeeper mock-up. It looks like they're training to load/unload a missile from a silo.
See here: http://en.wikipedia.org/wiki/LGM-118A_Peacekeeper