I decided to try bartamos suggestion about not double nutting. When I thought about the failures it occurs to me that the problem is that the double nutting is keeping the bolt from having any flex to it (although this would be very minimal). I'm starting to think that over a period of years and thousands of miles the repeated slight stress at the inside nut is causing metal fatigue eventually. Since nobody else is running into this problem, and lots of folks are using the Chinese replacement shackle kits the problem must be how I'm assembling it. Thanks for all the input. Only problem is that it'll be 5-10 years before I know if this worked.
David
