I was driving my truck on some forest roads and needed to turn on the 4wd. The A/C was on and blowing nice and cold and the hubs were already manually locked. When I turned the switch to 4x4 high on the dash something weird happened, the A/C sounded like it quit blowing and then after a few seconds it came back on again. The same thing happened again later during the day.
Today I pulled the front left tire and fender liner to get to my PCM. I noticed an open ended vacuum line with a plastic barbed connection just sitting there. It made me wonder if that is why the A/C acted like it did when I switched to 4x4. My guess is this had automatic locking hubs at one time and they replaced them with the manual locking.
Should those vacuum lines be plugged?

 

If you've gone to manual hubs then yes, you should plug them.

When you shift into (or out of) 4wd a solenoid actuates (~30 seconds IIRC) to "pulse" the hubs to lock or unlock the hubs (similar to clicking a ball point pen). During that time if you don't have an adequate seal in the ESOF portion (from the pulse hub solenoid down to and including the hubs) then you will lose your vacuum. That is what your HVAC uses to control the ventilation direction. The default setting (0" of vacuum) is defrost. This is why many people talk about their ventilation going to defrost when the turn the 4x4 switch.

The drawback to a diesel engine is that it does not generate vacuum like a gasser does so your vacuum pump is the sole source. When the solenoid shuts it takes a bit for the pump (not the highest volume) to build vacuum in the reservoir and the rest of the system before the ventilation will re-align to the setting you have selected.

That's probably more than you ever wanted to know but just wanted to give you the background.

The hub connection is a known place for the vacuum lines to deterioate/break so most of us cap them before that. This removes any issues with the connections and seals in the hubs.

 

Thanks. That more than confirmed my suspicions.