I understand your train of thought.
I do not do any off road driving, only local city streets and some freeway runs.
I believe I will remove and turn the half shaft on each side 180 degree at the inboard (rear end gear) side, one at a time, with a road test to speed in between, to see if moving them have any effects, negative or positive on the vibration.
The rear marker light lens on the driver's side is the one that came off due to vibration. I believe from this that the driver's side is the one that is causing the vibration. However, the half shaft ujoint that I recall being tight was on the passenger side.
If changing the position of the half shafts on the inboard side does not improve or eliminate the vibration, I will inspect the tight ujoint on the passenger side and regrease if not damaged to see if it loosens up and feels like each of the other 3 ujoints (that were all replaced at the same time).
Thank you for your help,
Andy