It sounds like you are describing the AFC housing. It does look kind of like a diaphram type pump.
The housing is were your boost pressure enters and controls you fueling rate per pound of boost.
If you have fuel leaking there, it is pretty much guaranteed that the fuel is coming out the little side hole containing a horsehair breather.
It that is the source your leak you have fuel that is pushing past an o-ring at the bottom of that housing.
Normally you will have to either pull the pump top off and have it repaired OR take the truck in and have a shop do this for you.
It's not a big job, but it takes an hour or so to remove the top, press out the old and press in the new one.
If you do not fix this, you will cause grief with the injection pump thru fuel and/or pressure loss... . not to mention it is not a safe condition.
Pastor bob.