You can check difflock is working by jacking up one wheel on each axle at the same time (chock others) then try 1st gear, if both lifted wheels are turning then your difflock is engaged (small lever to the left) you should also have an orange light illuminated on the dash with a H symbol on its side. If the light is not on then give the lever a good wiggle, if still no light you probably need a new switch which can be accessed easiest by removing the gearbox tunnel. You can use difflock in hi or Lo range but don't leave it engaged as you will cause transmission wind up especially if trying to steer sharp corners