What weight oil are you running? If you are running a multi grade, try switching to straight 30 weight. If that does nothing or very little to help the oil pressure, I would look elswhere, like Hot Rod Dad says about the rod and main bearing clearances. If just one is shot, that can make the oil pressure drop.
You could drop the oil pan and check the main and rod bearings, oil pump and clean out the sludge on the bottom.
I might catch some flack for this, and I wouldn't try it on a good running motor, but you could empty the oil, and fill it up with deisel fuel. Pull the spark plugs and with the ignition off, turn the motor over and try to "wash" the inside of the engine. To save the starter, you could even pull the truck around the yard in gear to turn it over. This might disolve some of the junk that may be clogging your screen. You might want to drain it and do it a second time while you are at it. When you have done that, drain the diesel fuel and leave the engine sit with the drain plug out to make sure it drains well. Then fill it with your new oil, and spin the motor again to get the oil lubed before you start it.
I said I might get some flack about it because if non detergent oil has been used, there may be a lot of buildup inside the motor and you may be loosening a lot of that stuff. Using diesel fuel will most likely disolve some of that stuff before breaking chunks off, but it sounds like you probably may be working internally in the end any way.
Keep us informed.
