All the rear brake swaps I've seen have swapped the entire rear axle rather than just the brake hardware. I'm not sure if that was actually required or not but it seems to be the common route.
That said, from what I've heard the factory rear Mk3 disc brakes are nothing but trouble. They apparently have pretty much the same stopping power as the drums, VW used them more as a marketing tool than anything. A lot of the import cars were coming with 4 wheel disc brakes, so VW needed to put a check mark in the box so to speak to stay competitive.
From the reading I've done frozen calipers and parking brakes are very common with the rear discs.
Sent from my HTC One X using Tapatalk
now Free