That's definitely a job you have to really want to do because it's very tough to get into federal law enforcement. Even if you don't reach the level of Marshall or FBI agent, you can still have a successful career in state or local government. One of my friends started out in the Marines as a firefighter, found out he was afraid of heights, and ended up serving for their military police. He now is a local cop in a suburb of Columbus. He does pretty well and has better benefits than I could ever dream of working in the private sector.
You're already ahead of the game if you have a direction you want to go. The biggest problem I had in college was a lack of direction. Just head in a direction and finish it. If you want to change careers, the biggest thing is just having the degree. Most of the time employers don't care what it's in as long as you have it.
For instance, I manage a small business for a living that specializes in the natural gas and propane energy sector. I have a degree in business, my employees have degrees in physics, journalism, English, and the other has an associates in accounting. The guy who makes the most money is the owner and he only went to high school, so go figure.