I think you can still get the Haynes service manuals from places like Advance that covers the vacuum system. Also if you know anybody at the local GM dealer in service you will more than likley find a room full of old service manuals that you can make copies of. GM has most info on shop computers now but most dealers still have the old books. I've got 2 different dealers here and if I can't find something they let me make copies from their old shop library.
I don't know it this helps but IIRC, there are three vacuum lines on the transfer case. One comes from the engine, one goes to the vacuum module that engages the front diff. Can't remember where the third goes, but they should be bundled together as the go into the transmission tunnel on the right side. Edit: the third line is the fluid overflow, ends up behind the right headlight
When you manually shift the transfer case, the vacuum switch on the case will engage the module in the engine compartment. The module pulls a cable that should engage the front diff.
It's common for the switch on the case to fail, but equally as common for hoses to be melted/split/fallen off. There are two electrical connectors on the console - one is from the T-case to tell you what gear it's in and the other is from the front diff to tell you if it's engaged or not.
I've got the Haynes S-10 manual and I don't remember seeing a vacuum diagram in there, but I could be wrong - it's been a long time since I looked.
I got your truck in the 2wd version. Good luck with the diagrams, I havent ever been able to find any of them. Checked the dealer shop books and all. If your vacuum setup on top of the engine is the same, there is a multi-connector where several of the vacuum lines converge. I tried to order said part and was left to 'fabricate"something of my own. have fun with it.