The old U Joint makers used to have a film out back in the 60's they put a std 7 inch saw blade on the drive shaft right at the u joint--they would spin it with it straight and put a playing card to the saw blade--it sounded smooth--they would increase pinion angle little at a time and when it was past acceptable pinion angle you could actually HEAR the saw noise starting and stopping proving what was actually happening at the u joint
a real eye opener!!
Those engineers were pretty smart guys