Sounds like I'm back to square one. Keep in mind, the symptom is very similar before AND after replacing the known-bad CSB. So by logic, if the same symptom immediately reoccurred, either the brand new CSB immediately failed OR the CSB is not the problem, or at least not the ROOT cause of the problem. The fact it immediately reoccurred is why we suspected the U-Joints being bad. I don't know much, but one thing I do know is that bad U-Joints can cause a CSB to fail very very quickly. It could also be that the U-Joints themselves are responsible for the sound under torque, and the broken CSB is just a symptom of the bad U-Joints.
I think the odds of a high quality replacement CSB immediately failing in the exact same way as the OEM CSB due to poor workmanship is pretty darn low. This was a Febi-Bilstien replacement, as near to OEM as you can get. I specifically sought out the highest quality components I could find, avoiding all-in-one kits that were half the price and purchasing each part from the best manufacturers. Anything is possible but...
As for incorrect installation, I don't know. We followed instructions to the letter but since those instructions (from Pelican) omitted the preload then we may have flubbed something. The procedure is fairly simple tho, I can't really think how we would have damaged it. It was inspected before and after attachment to the driveshaft and looked fine. Up until you bolt the bracket onto the car, it essentially just swings free on the shaft.