Yes. In most States, it is perfectly legal for civilians to own surplus military hardware including tanks, trucks, even planes, as long as you have all the right paperwork.
What's not legal is to have *armed* military hardware. Basically, you can own a tank but not a tank with ammo in it. This brings to question how Sergeant major Zack Carey was able to use the tank's main cannon on a number of occasions in the movie. For this story, a little suspension of disbelief on the part of the viewer is necessary, or the movie wouldn't be as interesting.
