Read a while back someone added bracing in the rear, a unibrace product which stiffened things up and reduced creaking but only on diagonal slops etc, could be worth looking into? stupidly expensive though...
I also have horrible creaking from the back, thought maybe my bushings were gone? (70k after all) someone also said the 3dr cars creak more? my mk5 doesn't creek at all and is 5dr (with 160k) but the gtd is 3dr and creeks, drives me nuts when there's no music on.