Failed Leaf Spring Inserts

[tjonesdfw]

Dealer says the leaf spring inserts have started to disintegrate and fail on our 1500 Denali.  It squeaks pretty badly going over speed humps, with any flex, and even when stepping on and off the tailgate or rear bumper... all worse when cold.  Unfortunately, they want $620 to replace them all which sounds excessive.  They say the original parts were not lifetime warrantied, but the replacement parts carry a LLPW.  Has anyone else had these go bad?