Without Alex’s tool or another if they exist, you must remove transmission and then tail shaft. Then must hone a bushing to some some .005 inch clearance. Likely with tail spline dead center in bushing.
I am not a machinist, but that is the general idea.

As for your assertion regarding load at cruise, perhaps you have a different problem.

I recall as a kid I had a 53 Olds with a .30 over 394 63 Olds engine with a LaSalle 3 speed and hurst shifter. Ran great, gave all the local Chevy, Ford and Pontiac street racers fits. Built by Speedway Motors in my hometown of Lincoln, Nebraska. Great bunch of guys in those days in the 60’s.

Had this very weird vibration and had broken several rear U Joints. One day while changing yet another with the rear wheels hanging free to rotate, while under the car, driveshaft out, I had my buddy spin a rear tire.

Only then did I notice the pinion yoke was turning in an ellipse and not a circle. Yea, guess I tweaked it a bit when blowing the initial U Joint. What did I as a 21 year old kid know about this stuff.

Perhaps under car in gear, engine running have a look see that everything is turning in a perfect circle.