My blazer did the same thing, and it ended up being the vacuum switch on the transfer case being bad, $20 part and about 30 minutes of work and BAM, done no more noise

 

This right?

 

That's the vacuum actuator located under the battery. The transfer case switch looks like this:


and is really fun to get to. Persistence and an open ended wrench will do the trick. Check to see if there's any fluid in the switch before replacing. Also, the ball on the end of the switch should depress easily and freely. When I replaced mine, the stocker no longer did this.

 

cant find the vacuum switch on thr transfer case... I do have a 3 button dash switch so there should one... What's the best way to get to it???

 

Diagram LINK

Is the best I can find at the moment without crawling under my truck and snapping a pic. I got to mine by lying underneath the driver's side, head by the LF tire. Reach up and around the transfer case. It will be on the top right if you're lying how I described. There are three vacuum lines attached to it.

 

Please do not double post your question.

 

Found the vacuum switch...
Replace it along with the actuator under the battery. 4x4 works as it should again.

On the vacuum switch I removed, the ball was stuck in. This must be such a common thing to go bad the vacuum switch was on the "HELP" rack at the auto store.

I also picked up a for sale sign while I was there!