OK, I was curious as the searches that I did at Autozone and Advance shows the same shock mount for 2000 - 2005. I agree 18k is a bit low for it to go. Mine made it all the way to about 55k before the shock shaft went through it.
Something I have learned owning a GXP. Many parts manufacturers assume that the GXP is the same as the other models of Bonnevilles. The shock mount may be the same or it could just be a case that Autozone and Advance aren't aware there is a difference. I have experienced this three times so far.