[ShaneHobson]

Put the shuddering car up on axle stands in anticipation of diff swap tomorrow morning.
Interesting observation, the parts car (with ex-Legacy 4.41:1 diff) has a limited slip diff at the rear, confirmed by jacking the back up, turn one wheel and the opposite wheel rotates in the opposite direction, but the shuddering car which apparently has a factory SVX diff doesn't do this.

Car is sitting on 4 x axle stands, nothing touching the floor.
Rotate rear wheel slowly and opposite wheel doesn't move.
Rotate rear wheel more quickly and the opposite wheel rotates in the same direction.

Very odd. Not terribly concerned, as this diff is coming out and may well be damaged due to having been driven by a 4.41 transmission for the past 5000 km.

But if anyone can shed some light on this I'm keen to hear your opinion.
Eve wild speculation is interesting.

Photo of the label and the diff attached.

cheers, Shane