4 wheel drive will not engage.

I have a 1998 chevy 2500 pick-up. I am having troubles with my 4-wheel drive. The switch is electronic and the lights change with the appropriate selection, but the front wheels are not engaging. With this, when I switch to 4-wheel there is a clicking in the front end at low speeds (below 10 mph) and then it goes away. Noise not affected whether I am going straight or turning. I changed the driver side CV shaft last spring. This noise is different than that(less clunking, more clicking). Any advice is appreciated.