Got 'em, and love them so far, although I haven't wheeled the X hard, and it's back at SLR getting tweaked. They're 2.5" and much stiffer than stock, but they absorb whoops and potholes like they weren't even there. They did require new mounts, etc. As far as I know, there are no Kings ready to bolt right up.
Soon, I'll post the rates, valving, etc, when I know the figures. Also, my qualitative assessment of ride quality. I know the shock we're going with in the rear is 9" long; 10" limited up travel too much.
King's reputation speaks for itself. I know some of us debated name recognition on a thread here recently, but when every other race truck runs something, well, the conclusion is obvious.
To elaborate... King's are pretty much the "be all and end all" of hi-perf race shocks! As Dave stated their reputation speaks for itself. The only comparisons are Sway-away, Radflo, Donahoe.
Unless you are ready to start custom fab you won't be able to put them on the X... I'm not even sure if they make a 2.0" shock (2.5" may be the smallest diameter).
If $$$ is even remotely a concern; look elsewhere.
Despite the fact that I'm running Kings, I'd have to agree with The Kartman here. People who know a lot say that all these companies, the ones that make real racing shocks, make a fairly comparable product. At one point, Spencer was trying to put me in Radflos.