I just bought a 02 XLT V8 a month or two back, and am in the same boat as you... Being a HP junky, I slapped a K&N FIPK kit ($198) within the first week or two. Noticed a very impressive sound difference and a slight HP gain. I also changed the plugs ($2.98x8), did a synthetic fluid change($3.99x6 quarts), low temp thermostat($14) w Redline water wetter($10), and itching for more. My next step will be exhaust, most likely a Magnaflow welded up to the stock system.....$100-$150, Unless I get a deal from my buddy on a custom 3" catback mandrel bent system, but he is being an @ss!!!!
I also have been looking into the granatelli "hot sparks" coil packs, which claim 15% fuel consumption improvement as well as significant HP gains. Cost is about $250
They also sell a MAF that increases air flow and should be good for a few HP...$285
Once I get that stuff going, then it's DEFINATELY a custom tune from BamaChips.
These explorers with the V8 have to be capable of some pretty hefty HP. Key is to get them open and breathing I think, and everything I mentioned above is a good way to do that.