Was the lifetime warranty stated back when you bought it or was this a more recent policy change? I know most people don't keep all their receipts, but if you still have it, check to see what it states. If not, 5 years is a long time and I'd bet they don't keep records that long so have no proof you bought it new.
It's hard to say something is a manufacturing defect after 5 years of use unless you often checked to make sure everything was torqued to spec.
I'm not trying to say it's your fault, but I can understand why they don't give away sets at someone's word over the phone without you providing more info. Good luck, though.