Low Oil Pressure

I have 2004 Silverado 2500 Crew cab with the Vortec 6000 engine (50,000 Miles). Today when I started it up the oil pressure was so low that the idiot indicator was dinging and telling me my oil pressure was low. I looked at the gauge and it was bouncing around from 10 to 20 psi. That is not right!. I shut her down, checked oil level, knew it was good cause I just changed the oil approximatly 500 miles ago. All looked well so I started her back up and now my pressure varies from 20-25 psi at idle, the gauge moves quite a bit while the rpms remain constant. I tapped the gas and I can only get 35 psi. What is wrong with this thing? Where is the oil pump located? Anyone have any trouble with this? Anyone have a schmatic of the engine so maybe I can replace the oil pump? Is the pressure read after the oil filter? Is the pump replaceable without pulling the engine? What is the recommended oil pressure?
Any help appreciated.