On the front of the gearbox, beside the bottom end of the red/yellow knobs, there is a "tin" cover over the end of the moving selector shafts for the hi/lo and 4wd. Chances are its full of gunge and rust etc. Remove the cover (I think its held by two small bolts) and make sure everything is free to move. Lubricate all of the bits before replacing the cover. You can do this from underneath.
ZD