You lost line pressure.
Both shocks on the "X" are toast.
0 line pressure means no fluid in the shock=shock piston hammering inside the housing=everything in the shock is garbage. Its right there in the RS6 literature.
So, if the line was the RR shock. Then you need RR and LF shocks replaced. Dont even let them tell you otherwise.
Edit: I would even go so far as to mark the two shocks so that you know they were changed. Its hard to tell if the Audi dealer knows the damage that was done. They may think your on crack or they may understand. But dont take any chances and keep on them.