clicking in front end

This past weekend while driving down the interstate i pulled of to the off ramp and started getting a clicking sound coming from underneath. It now wont go away and is constant and sounds likes it is coming from the right side front of the vehicle. It does go away when in reverse and 4hi and low. Any help would be nice. I already replaced the passenger side cv joint. 
I have a 96 S10 blazer
 
Thanks Richard

When going forward does the sound change with speed?? If the sound is constant through all speeds I would put the front on jackstands and spin the front tire with your hands to see if that reproduces the sound your hearing, to better trace the origin of the sound!! Let us know if that helps and what the result of the test concludes too!!

did do that and it only does it under a  load

its coming from the front differential.  the four wheel seems to be half engaged causing it. took the vacum off off the line and it does not do it in 2 wheel now but dont have 4 wheel for the time being
 

Then this definitely sounds like a vacuum switch problem at the t-case.