Past few weeks I have become an expert on drivetrains for my '04 A6 2.7t...which I never wanted to be! I drive far distances for work in my Audi, typically 3 hours one way on Mondays, then back 3 hours on Friday.
On one of my last trips, after about 2 hours of driving, I started getting a very bad vibration coming through my gas pedal and some in the seat of my pants. By the time I arrived at my hotel an hour later, the vibration was so bad that my leg was literally shaking.
To make a long story short, over the past few weeks the car has been in 2 shops, and here is where we are at:

Only vibrates during acceleration, worst from 35-60 mph. Vibration stops when I stop accelerating. NO vibration when braking, coasting, or in neutral.

Only starts vibrating after I have been driving constant for over 2 hours, typically on the highway.

My normal Audi shop is wanting to replace axles/cv joints. I am thinking that or motor mounts, but we figure starting with axles is cheaper (motor mounts on the 2.7t is a 15 hour job...).

Any other thoughts?

2004 A6 2.7t Quattro s-line automatic, APR 93 octane chip, Dunlop winter tires.

 

It sounds like you have an issue with (constant) velocity and heat generation (out of grease?) The shop probably is right on with replacing the cv axles. Do you have bad CV boots?

If the motor mounts were a problem, then you should probably have felt vibration at idle as well.

 

The boots look fine, and there isn't any grease leaking anywhere, which I think is why they are going with replacing the whole axle instead of fixing the boots.
Hitting the road again today for a couple hour drive - we'll see if she lasts!