I have a 2000 A4 Quattro and my back end sways - one when it hits a bump and esp. when it is raining. i can't take it over 40mph in the rain cause the whole back end feels like am going to spin out. Any suggestions on what it would be?

Bad rear suspension it sounds like. The only other thing I could think would be bald rear tires. When was the last time you changed your shocks? Also, is there a problem with your rear sway bar?

Welcome

check out the A4 section.

your problem could be a wheel speed/esp sensor

hvae fnu