It has a viscous centre diff as standard and open front rear. All tyres are similar tread and are the same (pilot 4s) breaks aren't the issue although I'm definitely going to upgrade those in the future (got my eye on some brembos). Oil levels seem good as well.
After a bit more research it is looking like centre diff is the culprit, namely the viscous coupling. Will strip it down and inspect and replace the coupling and any bearings and seals that need doing when I have time.
Does anyone have recommendations for where to get the bits I need that won't cost an arm or leg?