The 4 wheel drive is not working in my 2000 s10. when i push the button for 4 hi or 4 low i hear a click but i do not think the hubs are doing anything. What could this be?

 

not sure on the newer s-10's but on the older ones they had a vacuum controlled set up on the front hubs to lock them in and if the vacuum was not conected it would not lock in the front wheels so if you are hearing the clicking then either the 4x4 silonoid on the t case is not locking in or somthing with the hubs i think they went electric in the newer s-10's seen no one had given any ideas figured id try and point you in the right direction
prob the acuator or whatever it is called not engaging the front wheels or that 4x4 silonoid on the t-case