4.2L has misfire

I have an F150 2WD with 4.2L After replacing plugs & wires (yes I did check gap and double checked firing order) truck developed a misfire (happens randomly, acceleration, cruise at 50-55, sometimes not) Even replaced the coil pack. I haven't located any vacuum leaks, in fact vacuum reads a steady 18 on gauge. No engine code lights.???

Even Motorcraft plugs (the only ones you should put in that engine) can have a defective one from the factory. That is where I would start. Pull each one and give it a shake to make sure the ceramic around the electrode hasn't broken and is sliding down over it. Then check the outside ceramic for cracks.
My 4.2 when I bought it had just had Bosch plugs put in by previous owner. One of the electrodes had snapped off and created a huge gap that caused a pinging and misfire at highway speed for small hills and overpasses, but fine at normal acceleration.
Though you say vacuum is fine, check the line from the EGR valve. It is known to crack and cause intermittent problems depending on tension against the hose from different things such as engine torque twisting and air rushing into the engine compartment at highway speed.
If those check out, post back and I'll add to the list.

65comet, thanks for the tips. I'll be checking the plugs & EGR today. Will reply with results.

65comet. Couldn't find any problems with EGR, all connections tight, etc. I did however find a cracked plug (on the first one I pulled). I had used Autolight double platinum, & since I had to go to the parts store (O'reilly) I went ahead & got a set of Motorcraft thin wire Platinums. He said they were factory gapped, but I always check & sure enough they were gapped, .035, needed .054 for the truck. installed them and truck runs GREAT! Thanks again for your tips. I was not thinking of the plugs cause they were reasonably new.

Glad you're on the road again.