Cruze transmission problem (part 20)

I bought a second-hand 2011 Series II diesel Cruze as I needed a cheap commuter. However, since owning it I have noticed a persistent shudder, which I have assumed is coming from the transmission. It is most notable at between 20-40km/h and feels as if the whole car is moving side to side, which makes the stop-start of Melbourne gridlock almost unbearable. Is this a known transmission issue? If not can you clarify the transmission issues further? Also can you point me in the right direction of a contact at Holden so I can get the necessary diagnostics and repairs done?