I am trying to figure out where the red vacuum line (passenger side) attached to the back of the intake manifold attaches to on the engine. It has a split line, drivers side attaches to the fuel injector rail. The line I am concerned with is plugged, but I'm concerned that it should be attached to something. When unplugged it has lots of suction that seems to be needed somewhere to increase performance.

2004 Lariat 5.4

 

Is your truck 2 wheel drive? If so then that line should stay plugged and its for the 4wd solenoid.

 

Yes, it's a two wheel drive.
Thanks, it was bothering me, thought I missed something after replacing the intake.