You must obtain a five star wanted level, and steal one of the FBI agents' vehicles, to get an FBI truck in the game "Grand Theft Auto: San Andreas". The specific model of vehicle is called the FBI Rancher.
Gaining a five star rating will cause the police to be replaced with FBI agents. Forcing the FBI to chase you is the only way to get this truck as they are the only ones who drive it. Keep in mind that this truck cannot be gotten rid of easily. The spray shops in-game will not be able to repaint the FBI Rancher, and you will not be able to remove your wanted level.