Hi, I'm not new to Audi's as this is my 4th one and I can say I'm 7/10 when it comes to working on them, though I'm not a mechanic, but I've come across a problem that I've found only 2 people to have something similar but I did what they did and I still have the problem.

Car: 2001 Audi A6 Quattro 4.2L running on 255/40/18 ( only reason why I'm posting tire size is I'm curious if this may be the problem or part of it )

Problem: From 0-20 mph at 1800 or higher RPM, the rear of the car shakes, the violence depends on how hard I'm accelerating. If I can accelerate and keep it under 1700 rpm it doesn't shudder, but I live in a town with a crap load of hills, so thats rare.

However, it doesn't happen in reverse and once I'm passed 20 mph it instantly goes away and tranny shifts mint and cars drive likes silk from 20-120 mph. I've replace all 4 axles with OEM refurbed, all 4 from same donor car. Tires are brand new on brand new rims, high speed balanced, even tried 3 completely different sets 16", 17", and 18", no difference.

I'm located in CT, no vag-com access or lift, but I did put car on 4 jack stands ( ******* scary! ) and could not reproduce issue ( probably since theres no resistance ).

My thought is either a sticking Torque Converter Clutch or bad center diff? but I've dropped close to $2k already.

Any help is better than where I'm at now. Thanks and sorry for the novel.