It will fit but a lot of the design had changed from 98 to 03 making it impractical to do.
You will probably never pass smog inspection after the swap.
FoR, I am in Alabama wit is this smog you speak of. HAHA
Same here in Kansas, we don't have smog.
I choose Earthquakes over tornadoes being in Calif, but I am in Calif 50 miles from the second most polluted city in the US, By Year Round Particle Pollution - Home wood-burning fireplaces are illegal -- http://www.stateoftheair.org/2015/city-rankings/most-polluted-cities.html but 5000' above without a hint of smog.
Oklahoma, nearby has more earthquakes than Californis. I do NOT like earthquakes.
Your 98 explorer, if a 6 cylinder, is an OHV engine (push rod type) The 4.0 in the 2003 is a SOHC engine. If they bolt up you are going to have some problems with the PCM, etc. Electronically speaking, it is not going to be a simple "drop-in". And you definitely will not pass a state emission test, if your state has one.
Looking for a Used Explorer in your area?
CarGurus has 70,904 nationwide Explorer listings starting at $1,000.
Search Ford Explorer Questions
Ford Explorer Experts