I have a recently acquired a spec b 3.0 2006 pre facelift. When more than 180° of steering lock is used e.g. very tight corners or when parking at slow speeds, the front wheels shudder and skip as if it had a locked diff. I know it's a viscous centre diff and they should lock when traction is lost but I wouldn't think it would be this unforgiving and jolting in a standard road car especially when the front diff is open. Also there is a bit of whine from the transmission that I assume is related. I'm thinking it could be the coupling but I'm unsure and need help or should I not be worried?

do all the vehicle tyres natch brand/model and wear ?
Could undead be centre diff issue if tyres all as should be, it will stress the transmission & driveshafts severely if is ...

As a Spec B would this have limited slip diffs either front or rear or both.

I have no in depth knowledge of Subarus with them, though possibly our XT may have one at the rear, but the LSD on our Hilux was so fierce you could barely manually turn the rear wheel with the vehicle raised.

As above, these vehicles need very similar tread depths and tyre types all round.

You've done the obvious, ie checked all transmission levels, prop/drive shafts all in good conditions and there no brakes dragging  anywhere.

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?

the centre diff (viscous coupling) is fairly easy replace, they come complete with bearing normally as a unit, not that cheap, you best looking up part numbers and searching for good price from that .
I've had new oem units with bearing from 280 to 350 in past (not from dealer parts counter), not done a spec B so not sure how relevant that price range would be without looking at part data .