Just reading this and I am wondering this. Can you assemble without the driveshaft installed and Test for the judder? I am thinking if you assemble without the driveshaft and you still have the problem. Then your issue is with the engine/trans. If the problem is not present then the issue is with the driveshaft/ differential, or the Driveshaft / differential are an element in the causation of the problem.
I don't know if this makes sense but this is what I would try. I also like the hammer idea!
