By David Shepardson WASHINGTON (Reuters) -U.S. auto safety regulators said on Friday they have opened a formal safety probe into the autonomous driving system in vehicles produced by General Motors Co’s robotaxi unit Cruise LLC after reports of two injuries in rear-end crashes. The National Highway Traffic Safety Administration (NHTSA) said it has received notices of incidents in which self-driving Cruise vehicles “may engage in inappropriately hard braking or become immobilized.” The agency said while both issues “appear to be distinct, they each result in the Cruise vehicles becoming unexpec…