4WD won't engage

[picard]

Greetings,

 

I have a 1997 Blazer. Recently, I tried to engage the 4WD by using the push button control. All I got is a clicking noise coming from behind the radio area and it wouldn't switch from 2WD to 4WD. Found out it was a blown 20amp fuse, thank God! Anyways, it's done this before. Any idea what would cause this?

 

Thanks!