I'm a mechanic and I'm seeing more and more 88-newer chevy trucks coming in with rusting off U-bolts in the rear. The cradle that holds the rear end to the truck has two pockets that collect water and dirt and causes the u-bolts to rust.
I changed a set yesterday on a 1990 Chevy 1500 2 wheel drive and I was able to break the U bolts with my bare hands they were so badly rusted.
GM isn't interested in doing anything about it so maybe if more people start complaining, they'll address the issue! Please be sure you check your U-Bolts, clean those pockets out regularly, inspect carefully!