Ok... lets start at the beginning then
Does the truck drive fine in high range ? Can you use all the gears - 1,2,3,D,N,R,P ?
If the above is yes then put the gearstick into neutral high.
Push it across to the left into neutral low.
Can you hear something (the noise will come from the transfer box) ? A bit like a "clunk" as the actuator engages low.
Does the LCD display on the dash say "low" ?
This is the "thing" that's rotating to engage high / low :
Depending on how good you are with a multimeter, you can remove this and test it on a battery. It's dead easy to get to and remove. Giving it 12v to see if it rotates. In my case, this was working fine, but the ecu wasn't.
With regards to a haynes manual - download rave http://green-oval.com/joomla/index.php?option=com_docman&task=cat_view&gid=13&Itemid=29
But having said all of the above, i'd check to see if it's going far enough across, as you said you had to readjust the gearbox cable.
HTH
Gordon