won't shift into 4 wheel drive
 

i recently purchased a 2002 ford explorer (v6, auto4 wheel drive). it had
28k miles on it. when i purchased it i had it checked by a trusted
mechanic who said all systems, including the 4 wheel drive were go. now,
a few months later i press the button to have it shift into 4 wheel drive
hi or low, and nothing happens. then a week later it worked for a short
time. i took it to a transmission place and they hooked up the computer
to it and said the shift control modulater would not communicate with
their machine. so they took out my shift control modulater, and put in a
used one which did communicate with them, so they said it was my
electronic shift control modulater which was bad. they then put mine back
in and low and behold it worked again, for about a week. then recently i
was away for 2 months, came back and it worked again for about a week and
now nothing. how do i diagnose this problem. it sounds to me more like a
short in the wiring than the modulater itself, but i don't know. and also
if i could find a used one, should i put it in? and is this the correct
term for the part? it sits on top of the evaporator on the passenger
side, i ask this because every website i have looked on has no listing for
it at all. i don't use the 4 wheel drive that often, but you want it to
work when you need it. the chiltons manual i have says it is not a hard
job to replace the modulator, what do you think? thanks