The 4.0l SOHC engines had a poorly designed timing chain setup from 1997 to 2004, Explorers got this engine in 1997, Rangers in 2001.
Unless yours has been updated to the newer design the sound is most likely that one of the front timing chains is loose.
Only long term fix is pulling the engine and replacing the old system with the new one.
There is a timing chain in the rear of the engine, which is why it needs to be pulled out.
When engine is cold, loosen and then remove the fan belt from the crank pulley.
Start engine, battery light will stay on since alternator is not spinning
Cold engine can run without water pump for a minute or two.
Now you can listen for the rattle without fan noise or pulley noises.
Common place is behind front timing cover or passenger side rear
But if rattle is gone then it is a pulley on one of the accessories.
While it is not common, if timing chain slips enough it can cause valve piston damage.
But while it is loose the valve timing changes which lowers compression and will cause rough running at times, the timing chain tensioners run on oil pressure, the tensioners can get stuck and allow chain to get loose and then tight up at certain RPMs causing the "skipping" issues you describe
Last edited by RonD; 04-09-2016 at 10:06 AM.