While Alabama has rules regarding seatbelts and child restraints inside vehicles, the state has no specific laws addressing the legality of passengers or pets riding in the bed of a pickup truck. This article explains what the law says about vehicle passenger restraints and how that might apply to riding in the back of a pickup truck.
Alabama's seatbelt law requires all passenger car occupants and children under 15 years old to be properly restrained. In the past, the seatbelt requirement applied only to "front-seat" passengers. But a recent change to the law deleted the front-seat distinction. So, it would seem that under the new law, passengers can't lawfully ride in the back of a pick-up truck.
Alabama has no law prohibiting the transportation of animals in truck beds. However, reckless or cruel mistreatment of a pet can result in a fine up to $3,000 and up to one year in jail. In any case, the American Veterinary Medical Association recommends that pets be secured either by a kennel in the truck bed or by a harness inside the cab.