Testing is how you did it, with a small pry bar.
But vibration & noise could be coming from anywhere.
Are you loosing tranny gear lube?
Is the rear axle filling up with tranny gear lube?
How are the rear motor mounts?
If the clearance between the propeller shaft and its torque tube bushing exceeds .10"
The torque tube bushings (Front and Rear) and the oil seal should be replaced. Inspection
And GM film strip
scroll to slide 13, more slides below 13 also.
You can do it the hard way (pull the torque tube from the axle housing) or the quicker way install a Okie bushing
More info HERE
A lot of the times the reason there is vibration and noise in that area is not because the u-joint is bad.
Its because the the front and rear torque tube bushings are bad, making the u-joint to flop around and then it goes bad. Prematurely wearing out. Troubles and Remedies
Your torque tube bushings are press fit no dowel pins.