Well to elaborate, from reading, the main issue that points towards the u-bolts is when people add a taller or shorter block to the rear end, and reinstall the GM u-bolts. From what I've read they're only made to install once, since once they're torqued down they stretch. Going tighter can help, but if you loosen them up, you'll have to go tighter to stretch them a tad bit further than before. Everyone's heard that, right?
So that's what got me thinking, as a metal fabricator. We often get the same metals in our shop, but they don't always weld the same or bend the same, etc. I had a bouncy/vibrating rear end, and I guess that axle was shimmying around under those u-bolts. Tightened them and it worked wonders.
This also led me to think, when people were installing taller lift kits with new u-bolts, they were getting u-bolts from companies that probably ordered them from another company... So in theory, they might have no idea of the quality, consistency of manufacturing, or tensile strength of the u-bolts they have in stock, thus leading to an inconsistency in torque specs with each lift kit... Or in my case, in was with the GM stock bolts.
Sorry for ranting. Try it man, I hope it helps.