Flexible Propshaft Joint attached to Haldex. Vibration on accelleration

Hi I recently bought a 2012 (not MQB Platform) A3 170 Sport Back Quattro Black. 

I have Owned it about 3 months now) it has the infamious Vibration on Acceleration.  It comes from the rear and you can feel it through the driver seat. 

If I pull the fuse for the Haldex 4WD the vibrations stop and I can spin the front wheels only then. The running gear feels good and no vibrations or knocks.   Yay! We thought and garage I bought car from swapped the Gen 4 Haldex and Diff.   Sadly it was a used one and hence a question mark!

Sadly the noise was still there and after a lot of reading I checked all tyres are good  balanced front and rear and true round.   Found the Front two have small flat spots so I will sort replacements but still no wobble of the steering or alignment issues as tyres are wearing fine too. So here is my question.

I don’t think the garages mechanic swapped the prop shaft or the Flexible joint that attaches the Prop shaft  to the Haldex.  If I still pull out the fuse the Vibration is gone and drive train is silent  and no vibrations/knocks on turning or anything same as with original Haldex and Diff the car came with. 

Does this mean the flexible joint attached to the Haldex and Propshaft at the rear are good and it’s a possible faulty Haldex again?

Is it possible the Haldex is fine (as its is operating in 4wd) and the Rear Flexible weight balanced joint are effected only when Haldex operates with the fuse in not out.  Then why does it drive nice when the fuse is pulled out.  I though Prop Shaft rotates all the time. Am I wrong?  Confused! Propably as not a mechanic just reading up that all.

Sorry for long post appreciate any direction.  Going to swap flexible joint to see if vibration is solved but it’s the fact when I take out the fuse the car drives very well without vibration and FWD as it should. Could it still be the Flexi Joint (That Audi try and make you purchase a complete Propshaft) I am hoping  someone mechanical or who has experienced this can help and explain as I am new to Quattro systems.

Many Thanks in Advance