to free shafts if it is that (will be able to feel lever not working properly) yopu could try tapping the top frontmost part of gear box housing just behind tin cover ,or remove cover 3 x bolt and spray penertrating fluid into housing ,it works by the red lever pivoting on yellow knob shaft (which pivot point moves depending on what position yellow knob is )and moving outer shaft which has 4wd collar,once you get all three shafts moving it should work ,it is possible for mechanism to come apart in box but would require removal of box to fix