The weight applied as you describe reads like poorly executed work.
If you feel vibrations in the steering wheel then front axle issue. If you feel shake in seat its a rear axle issue.
The minimum they should do is switch resr tires to front and vice versa.
Drive shaft should be checked for out of balance with a dial indicator.
With a speed like 65 confirm if rpm or speed related by duplicating in different gears.
Dealership may not be using best techs since it is warranty work.
Their 65 mph specification for passing inspection is stupid- I doubt they'll put that in writing.
Browse this
http://www.hurst-drivelines.com/file...ration-FAQ.pdf