Thanks everyone. I had a knowledgeable mate take a look underneath. We checked the rear diff oil and it was there but he wasn't overly fond of the look of it and reckoned it should be changed (front diff was fine). Propshaft unijoints all needed grease and indications from the amount it took were that it needed greasing badly but they seemed to be fine, in a twist and shake test
He drew my attention to an oily looking front Bilstein (fair enough, they're about 12 years old and have been through at least 5 outback trips, including Mitchell plateau roads, two trips into the Bungles and just recently the Gunbarrell - they've been brilliant)
Also, he gave the wheels a good shake and reckons the rear dhs real might need a bearing looking at. It wasn't horrendous but certainly worse than his 110 trayback)
It turned out on my freeway testing that in fact the vibrations are worse under power at around 90-100kmh and disappeared when in in neutral at speed.
Driving home after greasing and it felt better but the vibes did return.
It's testing the wheel, balance, front shockies, getting an assessment on the next bearing.
