My new bullitt (20) had the vibration. My local fordtore (nottingham - UK) read this thread and very quickly tried a couple of the suggestions, but then went to a new drive shaft. The new drive shaft fixed it instantly.
I would suspect axle nut / hub first...but I had a very similar noise that was a sportline spring that had rotated in its lower rubber seat and was rubbing on the metal
Hi. I had a low speed squeak / binding noise.
For me It was the lowering spring not seated in the rubber seat (bottom end). That's an easy check, take the wheel off and you will see if that's the issue.