Right and Wrong. Headunits these days are building extra features like a crossover directly into them. However, you are wrong about eternal amps having passive crossovers in them. They have "active" crossovers in them. They are powered crossovers that are cutting out certain frequencies BEFORE amplification. passive crossovers usually have no power applied to them and cut out certain frequencies AFTER amplification.
Yes, you CAN find any set of speakers that is the same size or smaller than the 6x8 opening on the explorer and have them installed directly or with the use of a mounting plate/adaptor. However, the crossover that comes with most mid/tweet kits crosses the bass and mid freq out of the tweets and the tweet freq out of the mid. But they usually don't cross out the bass freq from the mid.
That's why I suggested getting the extra passive crossovers. If the speakers are 4 ohm, if I remember correctly, a simple 99 to 199 farad cap should do the trick. Run it inline with one of the speaker wires. It will cut out the bass.
Go to partsexpress.com to check out some caps you can order and google the words passive crossover design for more info.
Oh, and if you ever want to put an external amp on your door speakers, get one that has a crossover built in as suggested above and set it to cut out everything below, depending on the speaker, about 100hz or 150hz...