Hi Tim, your experience is almost identical to mine. Sounds the same and we've tried the same things.... However, I currently have it sorted by my dealership...
They identified the problem as being caused by the tailgate needing tightening against the striker. They also put some fabric tape around the striker, and, so far so good... not a creak in 2 months - including a long trip to Cornwall and back. I've declared it resolved, and at least have a very good idea where to go if it happens again.
Hope that helps you. Let me know!