Hello I'm new here anD my 4wd will not engauge on my 1990. Even when I swap the two vacuum line around on the diff it still doesnt work. Now on the passenger firewall there are the Vsv valves and then after the three in a row valves I also have another right next to them that has vacuum lines that are 3 way connections. I'm assuming vacuum isn't reaching my diff because it didn't engauge when I swapped vac lines. So what is this other valve?

 

http://www.off-road.com/trucks-4x4/t...tem-19259.html

Check for plugged lines and vacuum fittings.

 

Got it all figured out a few vacuum lines were swapped around in the engine bay.

 

Yep, that is the most common problem cause that I have heard. Shop or PO works on engine, disconnects some lines, hooks them back up and engine seems to run fine so they call it good.