I am here to tell you what I have experienced. I had a shutter developing over a couple months. It got progressively worse, as expected, until it reached almost exactly 40,000 miles, at this point mine was shuttering on deceleration from 50 to 45 and the whole jeep shook like and sounded like, it was going over a rumble strip when decelerating between 25 and 20! I crawled underneath and tried to find any signs of a bad u-joint and found none. So I pulled the rear drive shaft and the u-joint closest to the rear diff was frozen! I could rock it back and forth in one direction, but the other was rock solid like it was welded... I took the drive shaft to a local drive shaft repair business and had both replaced and had it balanced. It drives better than it ever has. Smooth as silk

Well as smooth as a diesel can
Anyone having any similar vibrations and shutters should pull their rear drive shaft and replace at least the u-joint closest to the rear diff. If you have any lift, it is even more likely to be the problem as the drive shaft angle is much steeper and it puts more force on the joint.
I am going to get larger yokes welded in and an over sized u-joint installed when I'm bothered again

and I'm sure it's just a matter of time...