The final piece of the rod was given to the druids of Dor'danil for safekeeping. I have no idea where they hid it. I'm sorry I can't help you more than that.
All I can tell you is to follow the road that leads towards the Barrens. The druids took shelter to the east of that road, but they won't be of much help to you: they were killed recently... it's something I don't like to think about much.
When you find the final piece, bring it to me and I will help you reconstruct the rod.
