I have a similar as yet unsolved problem with a 1993 pajero. Its not a problem with mine however may be worth a check on yours if the front hub lock works on a vaccum set-up. Check that you have not got a vaccum leak from one of the many air lines to the switches and down to the actuater mounted on the front diff. It is pretty easy to test the actuater because there are only two hoses with a blue and yellow marking. Dissconnect the blue hose, slide back the plastic boot over the actuater arm and by removing the yellow hose and replacing it the actuatre arm should move in and out (engage and disengages front hubs)Do this with the engine running GOOD LUCK