Have you checked your tire balance? I have the same thing happening to my ute right now at 45-60 kph. It only happened when I changed wheels a couple weeks ago. I haven't switched back to see if that was it for sure but I'm pretty sure that's it. Mine's not really bad but it's really noticable when you look at the shifter... I've got a shot gearbox mount and the shifter ocsillates at these speeds.