This thing is leaking vacuum and I'm trying to figure out what it is. It is below the LHS part of the 4.2 supercharged that says "Supercharged" (intake manifold) in the back of the engine.

 

Looks like it's called a supercharger bypass valve and not available separately. So I ended up smearing it with loctite gasket maker to shore up the leak for now.

 

Yes it is the S/C bypass valve. On the 4.2 engine it IS available as a sperate part - a bit expensive, but fortunately you can bench test it once its out to truly see if it is leaking.

If you get to the point of replacing it, get ready to replace all the other gaskets around there.