1988 Jeep Wrangler

The vacuum lines are for the Quadra-Trac vacuum operated shift mechanism on your front axle. There is a vacuum operated switch attached to the transfer case which routes vacuum when 4x4 high or 4x4 low is engaged. There should be 2 vacuum lines that go back to this switch from the front axle and then one goes to the engine and another should go to atmosphere. There might be several one way vacuum/pressure diaphrams that keep the system operating properly. If you are having trouble and it is not engaging the front differential then check the following items in this order.

1) vacuum operated switch on transfer case
2) vacuum lines for leaks
3) vaccuum diaphram at intake manifold

The colors indicate varying amounts of vacuum (in inches of Murcury (in. Hg.)) Or pressure (psi). There are color codes on the internet. Jeep and other subsidiaris carry vacuum schematics if you need something specific.