Why is it a "parts" truck?  Wrecked?

that we know it worked in.  But it still won't work. I got out

And if all of the wires check out ok there is a strong possibility that the
servo motor is just stuck.  It happened on both my '91 and '92.  I removed
the servo motor (had to cut the single brown wire going to the transfer case
and solder it back together when done), took it apart, cleaned it thoroughly
with carburator cleaner, lubed the bushings with motor oil, and lubed the
worm gears with vaseline (it's a long way to the auto parts store from
here--was all I had on hand).  It has been working fine since (6 months or
so).  Mark everything before disassembly so you'll get it back together the
way it was.  Those wires you mentioned that had no voltage to them might be
the indicators for 4H and 4L and would not have power to them unless the
transfer case was in those positions.  I should also add that the motor
brushes have little slots that the brush wires fit into to facilitate
reassembly.  Position the rotor between the brushes and release the wires
with a small screwdriver etc. and check to make sure they are not stuck.

The other method that has been mentioned here many times is to tap (gently)
on the servo motor while someone pushes the 4X4 button.  Sometimes it will
get it moving again.

Then, after you get it all working again, you can look into getting manual
locking hubs because the autohubs have a very high failure rate.  But then
you are lucky enough to have a "parts" truck available.

.http://www.accessconnect.com/superwin.htm