Idle is most likly a nackered or dirty ICV quite a common problem on the xu`s as for the judder at speed sounds a little strange. Do you get any movment in the steering wheel like un balanced wheels? Or is it like the engine juddering in the engine bay feel? The top left engine mount is quite prone to wear, maybe worth checking
ICV = Idle Control Valve , just incase lol
ICV = Idle Control Valve , just incase lol