You have to keep RCV's greased, but I'd still run them over a u-joint axle all day every day. Smooth power transfer when locked is noticable. And it's pretty typical for a u-joint failure to take out a lower ball joint.
I've seen a few instances of RCV shaft failure. It happens. They seem to stand by their warrentee. I've seen one RCV ball cage failure on a waggy D44 and 39" stickies.
And to answer your original question, JK RCV's are usually 4340. "Double Heat treat" is probably a marketing term. You can probably get a 300m jk rcv for extra.
2007 2dr X, 6sp, 35's on D44's, 2" lift, cage, hydro, etc.