Why does a driveshaft on a pickup truck with a carrier bearing use a slip yoke at the trans? Or how about a car with a rigidly mounted independent rear axle using a rubber flex coupler (LX). Because everytime a u-joint goes through its travel the length of the shaft has to change by a very small amount.
If you read the post that was linked to I included a picture of a factory style steering intermediate shaft. For those of us that turn wrenches for a living we have been seeing and replacing that style of shaft on GM products(among others) for years due to noise complaints. The problem is caused by a lack of grease on the sliding splines preventing the shaft from extending and compressing smoothly. Why do you think they stopped using u-joints on the front end of solid axle 4wd vehicles in favor of CV joints? It wasn't bacause a CV is cheaper to make, it is much smoother.
If you want a real simple easy to duplicate test for this try this. Put a rachet in a vise, attach 3" extension, attach a universal, attach another extension. Now put the two extension so that they are NOT in a straight line. Try to turn the extension without allowing either extension to move side to side as it rotates.
