[FatBoy]

I had exactly the same problem in mine. In my case it wasn't the rear bulkhead though that was vibrating, it was the trunk floor directly behind it i.e. the top if the step at the reaf of the trunk.
Have a look at the rivets that hold it down, mine weren't pulled down tight and allowed the panel to buzz. I suspect, although I couldn't be certain, that there is a slight process error in the assembly. It would appear that the trunk floor is put in after the rear bulkhead, which means the rivet gun can't get in properly to pull the rivets down when the floor is put in due to the bulkhead getting in the way.
As a temporary measure, I unscrewed the small panel in the trunk that gives access to the diff and put some thin foam between the top panel and the chassis crossmember that supports it. This gave enough tension to stop the vibrating.
Over the winter, I intend to take out the rear bulkhead and rivet the trunk floor down properly.
I hope that helps.

Paul