You seem to have "covered all the bases",

So I'll relate a job from waaaay back,

Seat van,

customer complaint was "It's haunted!".

Seems at random instants it tried to drive him into oncoming trafic.

Or some times when turning right it tried to go straight on.

So considering all he had done to it to fix it the only thing that it could have been was a faulty differential.

Which he thought I was talking horse s***.

Anyway he washed his hands of it,

I bought it for a big note if I recall right and moved it on at no mark up to a guy I knew was good with the VW boxes this was fitted with.

He had it on the road in a day,

faulty diff bearing on one side.

And he's driving about in it.

fine,

Until the original owner comes into me shouting and balling that I'd ripped him off!

He'd seen his van running about.

So I showed him my books with the sale of his van AT NO PROFIT.

And the mechanic had done as I suggested because he agreed with me that the box was in some way faulty.

Which it was.

So a bit of a long story but the owner of that van had paid so much and it had not done any good.

Leastwise for him,

his list of bills had left me with no other conclusion other that a faulty differential.

And his story sounds VERY like yours.

Now just to justify all this the whole point of a differential IS TO DRIVE THE ROAD WHEELS AT A DIFFERENT SPEED.

To turn left the right wheel has to drive just a little bit faster than the left to "turn" the car into the corner.

Now it might sound like a long shot but if you have a faulty diff at least one symptom of this would be uneven tyre wear.

or going straight on when cornering,understeer.

or oversteer,

or pulling left/right when going straight.

Bad news is how you test for this apart from stripping down the box and inspecting ALL bearings and parts...................