Time to drop the oil pan...
If there's chunks in there, time to find a replacement. If there's no chunks... then further diagnostics is required. I just saw a video this weekend, where the oil pickup on an Expedition was blocked an old oil-change-reminder sticker! I've also seen it plugged by foil (probably sloppy oil jug opening) and silicone from resealing (sloppily) oil pans and valve covers. And as suggested earlier... sometimes oil pumps can fail (not common at all with this engine family however).
Oil viscosity breakdown will also cause oil pressure to drop. This happens if you run conventional oil too long. Any idea how long it was since your last oil change, and who did it?
Also, as you drain the oil before dropping the pan... it wouldn't hurt to pull an oil sample, which you can use for additional info to decide wether it's a candidate for rebuilding.