Central Axle Disconnect, The vacuum operated mechanism that locks and unlocks the front axle. A S-10 uses a diaphragm on the fender well connected by a cable to the front diff. The Jeep mounts the diaphragm on the axle itself. The vacuum diaphragm is controlled by a switch on the transfer case. The Jeep uses a 4 line switch and the S-10 uses a 3 line. The Jeep axle uses vacuum to disengage the axle where the S-10 uses spring pressure.
A 205 would be extreme overkill for what you want to do.They are a big, heavy, gear drive case that is near indestructible. They also only have a poor 1.96 low range ratio. If your plans where to run a 500+ inch big block and 44's, the 205 is for you. For your small block truck, I think the custom cross member and mounts, another drive shaft, plus the added cost of the larger t-case would be allot more work and money in the long run.
Where do you live? Anywhere near Ohio? I have a NV3500 and NP231C that I could sell or swap if you are close. I think I still have a D30/D35C axle set out back as well. The D35 is sort of weak but it will give you a matched gear set until you find a better axle. A Ford Explorer 8.8" with disc brakes would be a good choice. Same lug pattern and all......
I'll try and post some pics in my album later this evening so you can get an idea of what you are looking at. Dan