Isn't that the vacuum line that goes to the brake booster? I'm not sure.

 

I was searching online and there is something like this that plugs into the rear of the intake manifold. Looks like a bitch to get at.

 

That does look like this one!



But if it is the source connection on bottom passenger side of the manifold, that truck is not going to run worth a s*@#$t. (El Mobile mechanco' is going to be coming back!) It supplies manifold vacuum reference to the Fuel Rail Pressure Sensor on drivers side - and if you have only one common FRPS, the passenger side is "PLUGGED".