Straight truck rear-end protection with liftgates

We are a truck equipment upfitter that sells and installs elevating tailgates on straight trucks. Recently, a customer who owns a straight truck with a van body and liftgate was issued a citation by a motor carrier enforcement officer for noncompliance with the rear-end protection requirements of 49 Code of Federal Regulations 393.86. We were told straight trucks with elevating tailgates are considered “special purpose vehicles” and are exempt from the requirements. Is this true?