B4000 4x4 not engaging

1994 B4000-hubs just rebuilt, tansfer case engages, frwrd shaft engages but hubs won't engage. What activates the hubs once the electronic shift is activated? Is it possible when the hubs were rebuilt the shop failed to connect a vacumn hose or electrical connection to the hubs? Anyone have a diagram or resources for understanding this system? Thanks!

If they rebuilt them, and now they don't work, take it back to the shop.