2000 blazer 4wd wont work

The light lights up when you switch to
4hi or 4low, the acctuator under the
battery colapses when you engage the
4wd but the front diff wont engage and
the front driveshaft wont turn when its
engaged. i replace the cable on the
front diff cause it was froze, and
replaced the vac switch on top of the
transfer case cause there was atf in
the acctuator under the battery. what
else should i check?