I think everything has pretty much been covered Mark. Check the brake calliper bolts, pads moving in callipers, wheel nuts, yep seen that. Also the bushes that locate the axle fore and aft. Prop shaft has been mentioned but it would not 'know' you are turning and not moving forward, although it would be turning slightly slower than the front. Shock absorber mounts can give you 'clonks' as well.
If you are convinced it is in the axle, you could remove the rear prop shaft, lock the centre and try it in FWD only, this would take the driven load off the axle, if you still get the clonk then it is unlikely to be in the axle as there is no load and more likely a bearing not tight enough, the play showing up as the weight shifts when you corner or, it could be a bush?
Can't think of anything else at ATM.
regards
Dave