P0300 difficulty

2009 Sierra 4.8 engine, bone stock, 290,000 Km. Started running rough and set P0300 code. Scanner show misfire counter for cylinder 3 & 4 only. I had coils, wires and plugs on hand so changed them on both cylinders, no change. No vacuum leaks. The scan is showing voltages for the O2 sensors in the correct range but both STFT and LTFT show zero. Are O2 sensors bad or is it a wiring issue. If the O2 sensors are bad why only affect 2 cylinders? I?m stumped.

How is anybody else supposed to be any less stumped than you are when we not only can't lay hands on the actual vehicle, but can't even see a log file of it doing the things you say it does?

IF you do the basic diag, plug wires, plugs, etc. You can try moving injectors to see if the miss follows. If all the basics check out than it will point to a cam with some wiped lobes.