engine misfiring

[Dennis94XLT]

My sister has a 96 Explorer - standard engine with auto trans and is 2 wheel drive. It runs well until it shifts into 3rd or 4th gear and then it acts like the engine has a bad spark plug and feels like it's misfiring. It is especially sensitive to doing this when the RPMS are in the 1000 range. I have replaced the plugs with new dual platinum plugs and it improved the idle dramatically, but did nothing to fix that engine miss. I checked the plug wires and the coil pack - nothing looks obviously wrong with either. I also couldn't find any vacuum leaks.
The engine never misses in first gear or second gear and also never misfires in any gear if I really step on the gas to push it. I can't figure this out. The code reader I bought says no errors are being reported - even with the code reader plugged in monitoring the engine during the time it misfires.
Does anyone have any ideas? I have a lot of suspitions but I don't want to buy a lot of new parts if I can find the one bad one. The truck has 69k miles on it and has been babied.
I have tried putting it in gear with the brake on and then stressing the engine - it never misfires when I do that but instead only misfires when the RPMS are low and the trans is in the last few gears - and I press the gas to accelerate slightly.
Anyone with suggestions please answer back.

 

[Alec]

If it is indeed an ignition problem the next things suspect would be the plug wires and the coil pack.

Though if in third or fourth gear (and at speed), most gas engines would run rough at 1000 RPMs.