Scoll down about halfway
through this link...it's from my '96 Sonoma 4x4, but I think it should be about the same as a '93 (which I owned many years ago...but way too long past my memory). Big hose comes from engine; goes into the one-way check valve. From there, it splits 3 ways. One line (5/16", I believe) goes to vacuum reservoir ball. One goes to the transfer case vacuum switch. One line reduces to that tiny little line that goes into the firewall for HVAC controls.
I would just replace all those rubber lines under the hood, the one-way check valve, and the lines going to and from the transfer case vacuum switch. Also check the vacuum reservoir, make sure it's not full of t-case fluid. If it is, then the 4WD actuator under the battery tray is probably also full of t-case fluid, and possibly the actuators inside the dash. Vacuum system in these trucks is a pain in the a-hole...been through my two trucks 3 times in the last year, fixing vacuum-related problems.
And a word to the wise....stay away from the Dorman t-case vacuum switch. I put one in my Blazer last year, and it was an epic fail. Not only did the 3 tubes on the top not
seal well around the rubber cap/junction thing (giving me no 4WD to start the winter), it let loads of t-case fluid into the vacuum system. The vacuum reservoir was about half full of t-case fluid. I like Dorman for a lot of things, but the t-case switch was a piece of junk.