I'm sure the wires would be fairly simple, just be sure you label
things so you get the new ones on the right plugs and coil location.
As to the Plugs, they are fairly simple also but the ones on the
passenger side, esp the ones toward the back, are most easily removed
by working thru the wheel well opening rather then up top.  Some
people can do it without removing the wheel other like to remove the
wheel to have more room to work in.  As I recall the farthest back
plug is more of a "do it by feel" rather then being able to really see
what you are doing.  I recommend Motorcraft Double Plats and most
likely you'll never need to change them again while you own it.

Somewhere between difficult and very difficult.

It's been a while since I've done it but the plug nearest the firewall on (I
think) the passenger side can only be accessed through the wheel well.  I've
done it without removing the wheel but it might make it a bit easier if you
remove it.  Make sure you have different size socket extensions, a universal
(may not need), and plenty of band-aids.  The plug in the middle may also
try your patience but is not quite as bad.  Keep trying different
combinations of extensions until you get one that fits on the plug and can
be turned.

I always change my plugs one at a time so only one wire is disconnected at a
time--no mixing up the wires.  Same thing when changing wires--one at a
time.  I like either the Bosch or Ford plug wires.  I recommend that you do
not buy the cheap wires as they will cause you grief the next time you go to
change plugs.  Also, I spray a little WD-40 on the plugs before installing,
both on the threads and the ceramic part.  It makes it easier to thread them
in and get the wire off later without the wire getting stuck and coming
apart.

I do not find it extremely difficult but I do not thing the Ford
Explorer is a good car to start learning how to change plugs. You have
to go through the wheel well after jacking the front end up a little.

It is very easy to crack a plug when changing the plugs on the
explorer due to the fact that you need a universal socket and
extension.  BE CAREFUL.  Cracked insulators can look perfectly normal
and be very hard to detect when you crack them.  
So be real careful on how you apply torque.

Good luck.  You can do it but be CAREFUL !!!

Sam in Raleigh.

Ditto on the passenger side plugs.  I'm able to crawl under my '93 4
door XLT, 4.0 liter v-6, without jacking it up; I use a foam rubber mat
on the ground to keep it comfortable, since I won't fit while laying on
my creeper.

Ditto on the various sizes of socket extensions and a universal  socket
(or possibly wobble extensions).

Ditto on the bandaids.  Wear gloves.  You'll be working blind on at
least one, maybe more on the passenger side, and they will probably
require some force to break free.  If you can't get it loose, try a
cheater on your ratchet, but be careful or you'll break off the plug.

After you do it once, you'll probably wonder why you didn't take it to
the shop, unless it goes well for you.  Or you'll feel so good about
saving $100 that you won't mind the busted knuckles, and extra trips to
the tool store for the right combo of ratchet, sockets, extensions,
etc.