The J42640 is a little tool that you stick in a hole in the steering wheel to keep it from turning:
You could probably find it at an Autozone or somewhere like that. But, since you have a 2000 and the tool is for 2002+, don't worry about it. It's really just to make sure your kids don't jump in there and turn the hell out of the wheel when the shaft is removed.
I did it on our old 2000 Tahoe and thought it was fairly simple. Took about an hour since it was the first time and everything that could get stuck got stuck. Patience is not one of my strongpoints.
In tspain's post...
the first picture shows the bolt connection at the shaft under the hood. It's easy to get to, but can be a little hard to undo since the bolt is exposed to the elements. When the bolt is removed, you have to pull the two sections apart. Again, kind of hard to do if they're stuck real bad and it's hard to get a good grip on them anyways.
The second picture shows the shaft as if you were looking through the firewall toward the front seat. The bolt is not clearly indicated on the drawing but I tried to draw an arrow to it and attach a copy of tspain's picture....it'd probably be easier to look up under the dash to see where it is.
When that's unbolted, you have to pull the entire shaft out through the inside. There's a rubber boot at the firewall and the shaft will telescope but it'll eventually come through. It's a tight fit.