As I recall from what I read the main things are..

- it's hard to get to
- there is a pin/pintle/rod inside it that you want to make sure you
don't let fall out as you take it out.  I'm not sure if it will come
out as you take the mod out or if it sticks in the bore of the tranny
but just be careful that you are ready to catch anything that comes
loose.  

I had mine done at a garage so I'm not speaking from experience but
other then being hard to get to (you'll have to remove the heat shield
assuming someone didn't already remove it and throw it out like my
shop did.  I made them pay for a new one.) and it's still pretty tight
looking between the tranny and the exhaust system.

I also have not done it personally but someone here posted how they did it
perhaps a couple of years ago.  They removed the seats to reveal an access
panel.  Remove the panel and the modulator is easily accessible.  As Ashton
pointed out there is a pin in the middle (so I've read) that you must be
very careful not to lose or damage.

I thought I had it bookmarked but I can't seem to find the post right now.
Perhaps a Google search will reveal it.

I went ahead today and replaced the transmission modulator on my 1994
EB 4x4 Explorer.  Thanks to help her I was made aware of a service
panel on the floor above the transmission.

I removed both front seats and the center console.  I then pulled back
the carpet on the passenger side starting from the passenger foot area
until I had enough back to expose the center area where the service
panel is.  The service panel is merely a metal plate about the size of
a 8.5 x 11 piece of paper.

I was then able to reach down to the transmission on the right side to
address the modulator replacement.  There is not a lot room.   Just
enough room to get a small 10mm wrench in to remove the 2 nuts.  Once
the first nut was removed I was then able to push back the heat
shield.  The second nut removed the stud from the transmission and the
hold down flange that secures the modulator in place.  
I then was able to merely pull the old modulator out....but very very
carefully paying close attention to the small metal pin approx. 1inch
long that acts as some type of push rod.  

I cleaned the pin and put it in the new modulator and then carefully
installed the new modulator paying close attention to the pin thus
kind of holding the modulator up until the very last second before
being inserted into the transmission.
The new modulator popped in and was obvious when it was in position
because of the popping/click noise it made when inserted all the way
in the hole.

Reconnect the vacuum line, screw in the stud down by the nut  with the
retainer flange and then push the heat shield onto the stud and put
the final nut on.  

Now place the service panel cover back in place and screw it down with
the 4 screws.  Please note that when you are removing the panel be
careful not to pull up one side more than the other as you will bend
the panel making it not seal properly when reinstalled.

I performed the replacement and did not reset the computer.  Maybe I
should have but I did not.   I test drove the vehicle and noticed a
big difference for the amount of gas pedal that is required before the
transmission will downshift ..even when going up hill slowly the
transmission shifted effortlessly without having to give more pedal
pressure to cause the downshift.

The symptom I was having that caused me to replace the modulator is
this.
The transmission will not go into overdrive until a lot of time has
passed or I put the transmission in neutral while driving down the
interstate and revving up the engine and then putting it back in D and
after a second or two the transmission will go into overdrive.
I have noticed that my explorer tachs real high which I attribute to
the lack of overdrive.  Even driving around town I am experiencing the
high rpm even though the transmission is in D.

I will take the vehicle out on Sunday and see if the modulator solved
the overdrive problem or not.

Thanks everyone.... I will  post an update about the overdrive.

Sam Cederas
Raleigh, N.C.