Best way to diagnose is to use VAG-COM (or VCDS) and scan the ABS module after the ABS/brake lights come on (but before turning off the engine). This will likely tell you the cause of your issue.

I've had a multitude of ABS issues on my '98 A4:
-ABS module bad, needed to be rebuilt
-ABS still didn't work, due to both front sensors being bad
-ABS was acting funny until the ABS/brake lights came on
-turns out a ring was busted on the front. replacing that finally allowed functional ABS.

The ABS tone rings, which (at least in the front) sit on the CV joint, are technically part of the CV joint itself. However I've replaced only the ring before (pulled it from another axle).
I may have an old/spare axle I can pull the ring off of but it would only be for a 96-2000 1.8T. So if your car is a V6 or 2001 1.8T this won't help you.

 

Ok ok... Really stupid question.... U say scan the module... Is that different location than scanning for CEL codes, under the drivers side dash?

 

Same port. If you only have an OBD2 scanner then you won't be able to access the other modules. Audi would more likely call that port a "diagnostic port" because you can access other modules in the car: ABS, airbags, lighting, central locking, security, etc.

So having the right cable and the right software (VCDS by Ross-Tech) will allow you to access those.