[Puglife]

Being quattro it wont be drum rear, but discs all round instead.

With back end jacked up, grab the wheel and tyre on the outside with hands on horizontal plane. Try moving wheel by a twisting motion (eg. right hand towards you, left hand away then back again). If theres play there its likely to be tie rod play.
Do the same but top to bottom of the wheel and that will show play in the balljoint and bearing.
Audi (and VW) bearings are a clamp fit, without the clamping pressure from driveshaft and hub nut they can have play them but a bearing would give a rumbling or groaning noise when driving and normally show up when spinning the wheel.

My guess would be balljoint, the rear tierods have a small tolerance of play which would allow the strut to move forwards but the angle of fitment would negate movement backwards or at least limit it if theres play in the balljoint.