Jumping between SOME agencies can be easy, or so I've heard - a few of the NPS rangers I work with spent some time with the Bureau of Land Management and other DoI/DoA agencies. If you're into the idea of law enforcement but also want some of the forest/park ranger stuff, consider looking into LE positions within the Forest Service or the National Park Service. At least on the NPS side, you'll do law enforcement stuff, but it won't be the same as regular local/state law enforcement. Just a suggestion. 
