if there's a serious problem with the t-case not operating the vac switch correctly, and you're considering spending money on it - don't do it. just get a posilok cable and ditch the vac cad once and for all.
https://dodgeforum.com/forum/2nd-gen...installed.html

 

Now that I think about it, you really need to verify if the TC itself is engaging to 4hi or lo. Forget the vacuum right now. If it's not, then even a Posi Lok won't help.

 

TC engages only when i switch the 2 Vac lines (black & red)@ the tranny.
Thats why I changed the Vac switch
I think the shifter is not engaigng the TC to activate the VAc switch. Or maybe its internal. I'll take the switch out & put it back in see waht happens.
Any one know the vac line setup 4 the plug for the switch?

 

The vacuum has nothing to do with whether or not the TC is actually shifting to 4wd. The vac is only for the front axle engagement. You should SAFELY raise all 4 corners of the truck off the ground. Put the TC in 4hi or lo, and see if the front driveshaft spins.