After my next dig into the truck and If I can't find a specific spot then I'll try the Centramatic.
I've had this same problem with my 2015 and posted the problem on the Forum (Driveline Vibration) back in that year. You've had more luck with the dealers than I have. The Chrysler service rep ended it with "operates as designed". Wish the salesman told me that before I wrote the check. I think you are on to it with being in the rear end. I haven't had the time to pull the axle shafts and check them. Did they ever do that for you? Did anyone mention the vibration dampener mounted on the side of the pinion area? Never had a truck with one of these and I don't see them on other makes. To me it indicates they have an issue in there. Once I get freed up I'm either going to change the entire rear axle with something from '03-05 or trade it. I too, travel at that special spot of 70mph. Recently replaced all 4 tires and it's even worse.