1998 Ford F-150 Engine Pinging

I have driven the truck approximately 10k miles since I got it. Since day 1 the truck has ran great, however had a light pinging at WOT on steep hills. Since then, the pinging has become alot worse. I gave it a tune up, and ran 5 tanks of premium with fuel injector cleaner. I had the truck scanned and came up with code p0303(misfire cyl 3), p0171(bank 1 lean), p0174(bank 2 lean) and p0401(insufficient egr flow). So I cleaned the EGR valve, checked the dpfe sensor which seemed to be within specs, and cleaned the air sensors inside the intake tube. Reset the codes and the only one it is throwing now is p0401. I have heard replacing the EGR valve sometimes won't make the code go away, so where should I start?