nene, what oil have you been using? Sounds lifter related. They don't commonly fail on the 4.2's, rarely but occasionally. How long did you let it idle for? If it wasn't such a pain to take off I would say pull the valve cover and start it up and take a look, otherwise I would start it up and drive it at fairly low RPM's once around the block and see if it gets better. Maybe slowly failing and a bit of increase in oil pressure may be all it needs to bring it back to life. I asked above as oil quality can make a huge difference.