J1708 (open collector RS485) at higher baud rates than 9600 bps?
If the Wikipedia article on J1708 is to be believed, then it's only J1708 if it's going at 9600 baud.

You may be able to make it work over short distances at higher baud rates, but I doubt you'll be able to make it work over J1708's specified 40m distance -- and regardless, it wouldn't be J1708 any more.