Following-up on this problem, the vibration was diagnosed as worn CVs by my mechanic.
Had both sides' joints and shafts replaced and it now drives a treat. Drove from Brisbane to Bundaberg tonight with no sign of the vibration returning.
At a guess, I'd say they were the original items, so at 260,000km, I think I got good mileage out of them.
Thanks to everyone who replied with their ideas on the issue, much appreciated
