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I doubt very much the cable stretched - do you carry a lot of heavy loads on the tailgate?
Take one cable off and compare it to another truck where the gate seems level - I think you'll find that something else has changed/moved/stretched/bent.
I've had to replace some that rusted and broke, but I've never used anything but OEM replacements. From FTEPartsGuy - $16.63 for the right one, $15.52 for the left one (no idea why they'd be different prices...)
No so much stretch, however cable will, but more slip in the fittings or break a couple of strands. Cable doesn't store energy, shock loads have to transferred to something that will give, some times it's the bolts, more often it's the weak crimped on fittings.
Doorman/Help make replacements for a lot of applications. They run about $15 each. The last OE one I purchased were about $50.
Check to make sure the little plastic hinge bushings are still there. In lieu of that, you either have the wrong cables, or something else is out of whack.