Is this the vacuum system check valve for the AC??

 

its one of several. i see another one just upstream of that one (towards manifold), and another one right at the manifold.

 

Looking at my truck and at your pix the line at the A/C lines supplies vac. to the inside of your cab, the line thats goes to your cowl is just a vac. resiveror, the one that T's off and comes around the front of the moter is for your cruise control and the one that Y's out and goes behind the intake goes to the Transfer case shift valve this is how my 97 is. I also noticed a one way vac. valve under my evap siloniod that is on a line and just stop thier. I'm going to look at it tomarrow in the light better to see if thats the way it is supposed to be or if it is broken off from someware.