I may have to take you up on that offer! I've got a 260 sitting under a cover that I'll be starting in december. This diff never cross my mind and I see the things everyday! I'm an apprentice at holden and they get thrown with minor issues so there very available. What motor and box are you using? At the moment I'm keeping my options open, its really going to come down to overall price. Although.. there are a few ls1 warrenty motors stashed away!
If you need any info about the diff or other parts let me know and I'll try my best to find out.