GM-GMC: heater, heater core, different designs

Question
2004 hd chevy antifreeze leaking from rubber outlet from heater core

Answer
Sounds like your heater core has sprung a leak.  This is a fairly major undertaking, it may not be something you will wish to tackle yourself, I believe you will need to remove the dash assembly to access it.  If you are planning on trying it there are two different designs, visteon or delphi, you will need to determine which one you have.
Hope this helps.