The RS4 in B5 guise is a true classic.
You've got the same engine as me (AKN). Great engines, and will easily top 250k miles. Renew oil (Castrol Edge 5w30) and filter every 10k miles and the timing belt every 80k or 5 years along with a genuine Audi thermostat and water pump and it will run forever.
Back to your rear suspension. Try renewing the rubber bushes on the ARB. It's a cheap fix and would be my first port of call. Also check the coil springs. The V6 isn't a lightweight, so not unusual for the rear springs to break and lose and inch off the end. The links to the ARB do suffer, so that would be next on my list. You could also check the bump stops on the dampers.
At 100k, I doubt it's the bushes just yet, but it is 14 years old, so anything is possible.
Useful site here:
http://www.audidiy.com/b5a4.html