Make sure the front output shift rail is moving back and forth when you operate the shifter, there’s two rails that shifter handle controls, on is low/high and the other engages and disengages the front output shaft. If the rail isn’t moving it could be as simple as the fork or rail being gummed up or something could be broke too. The linkage bar that controls the rails could be worn too. Mine was, I had to weld the holes up and re-drill them.
This link should help you understand what’s going on in there.
http://67-72chevytrucks.com/vboard/s...d.php?t=469726