You said they held up the 440 no problem. Was that right after the 440 was installed or having you been driving around? obviously a 440 is WAY heavier than a /6. I can't see how they would hold up an RB effectively.
Something changed obviously, the trick is to figure out what.
Maybe someone did swap them when you weren't looking. Don't laugh, stranger things have happened.
Maybe they have been "sprung" beyond their limit. <--<< This gets my vote all other things being equal.
