Leaf spring shackles

[JTS71 Mach1]

Part of the problem is even though the springs are only 10 years old. Were they the correct lbs rate springs to begin? With so much of the aftermarket / replacement parts not being anywhere near the quality / specs of the OEM. I've seen many a spring that was incorrect. Whether it was due to being the wrong length, size, strength, or a myriad of other possibilities. Just cause they say it's correct doesn't mean they know their azz from a hole in the ground. Once they've got your money, most don't care if you ride height is correct or not, sorry to say but ends up being the truth more often then not. 

JTS

As for finding suitable plastic / nylon. Walk through WalMart, and slowly look, and study things. You'll find something to make them out of.