Rear legs, pelvis and tail injuries are the most common to dogs when HBC. It seems the dogs are quick enough to turn to run away, but if the whole thing happens fast enough they get the rear end smacked.
Road rashes are very common. Every time I see one I think of what must happen to a motorcycle rider when they lay the bike down. One good reason for leather.
The behavior is very common for tail injuries - either fracture, dislocation or sprain. If there is feeling and circulation in the tail, all should be OK.
Without examining this guy, I couldn't tell you for sure whether or not I'd strongly recommend X-Rays, but always a safe thing to do.
Doug, DVM