You know I have been saying the same thing for almost a year, and I have been to the Jyard probably 100+ times to look for the solenoid, and I always forget to pull one...
From what I've seen looking at my own column, the solenoid mounts to the tumbler almost by sliding on.
As for the tumbler, I haven't ever replaced one on a car, but I used to replace them on residential and commercial doors.
You should be able to get the tumbler off by removing some type of retaining ring from the cam that is behind it. Once you get it out, depending on if it is shrouded or not, you should have access to the pins. If the tumbler is shrouded, it will make life much easier, as you wont have to worry about pins falling out.
If there is no shroud, be careful, as the pins will spring out all over the place if you do not cover the pin holes.
To remove the tumbler from the shroud (if there is one). Place a dowel rod or pipe that is the same size at the rear of the tumbler and use it to push the tumbler out.
Re-installation should be the reverse of removal.
If pins pop out, etc... Stick your key in the tumbler and place the pins, spacers (if any) and place them so that they lie flat when the key is fully inserted.
If you get a tumbler from the Jyard, try to grab up as many pins (from other GM cars) as you can in case you need to replace worn/damaged pins or happen to lose some.
I'm not sure of the type of pins GM uses, but most tumblers use pointed (raised center) pins, which can wear flat over time.
