Hello I have a 98 chevy blazer when i go to put it in 4wd the light comes on but does not work and i hear a clicking noise from under the truck, when i switch to 4 lo the truck jerks forward than starts to idle real rought any ideas why this happens .

 

Have you checked for vacuum leaks in the vacuum hoses at the trnsfer case?
Sorry, I see it's an older post, did you get it fixed?