you will have to have a look underneath there are 3 shafts poking out of output housing one nearest gearbox is worked by red knob lever hi 2wd ,low 4wd ,outer shaft that often sticks is behind tin cover ,and center smaller one which has a hole in it which a pin that goes into shaft when yellow knob is up and out when down the center shaft is a pivot for the other 2 shafts when pin is in shaft 4wd cant be selected in high ,when red lever is pulled back into low small shaft should be pulled back so that pin from yellow should go into the hole with kno up ,your linkage has probably broke or come adrift