Not sure if you guys found the cause but I was having similar symptoms. I took it in to Jeep and they said it was normal for ECO driving. BS...I traced back to when I first noticed it and it was my after my rock slider install. I thought" no way could I be getting a drive train vibration from sliders...it must be coincidence". Then I mentioned my sliders in another thread and how they were attached (to the frame AND pinch seam). I didn't realize that the new JK's have rubber isolators between the frame and body allowing the body to flex and move some. By attaching the sliders to both the frame and the pinch seam, I immobilized the rubber isolators.
I removed the sliders the other day and my vibration went away. I can still barely hear it at times but I can no longer feel it. Hope this helps someone else.