clicking noise in sharp turns in 4wd

I have a '91 Vitara and when making sharp turns with 4wd engaged a clicking noise seems to come from around the wheels. Could this be the hubs not locking properly? Can anyone help me with what could be causing this?
 
Thanks!

G,day Anna, worn C.V. joints will click when loaded up. ie- turning on dry or hard surfaces while in 4WD.To reduce wear, shift out of 4WD when not required. Leaving hubs in will also add to C.V. wear . Good luck,Tim