I'm having problems with my 4x4 not wanting to disengage completely back to 2 hi in my 2003 blazer. The transfer case clicks over, push button switch says 2hi, the computer thinks its in 2wd, and it drives like normal. But I believe the front axle is not completely disengaging. It's makes a very loud whining and grinding noise from the front end but if you click the 4hi push button it instantly goes away.. It does not make this noise all the time! As of now just once every few days. I managed to drive it for a week (around 300 miles) no problem then boom, the sound is back. I've replaced the transfer-case vacuum switch, front axle switch, actuator cable, actuator itself, and both differential and transfer case fluid changed. Any ideas?

 

This is a complete uneducated shot in the dark but could it be something to do with the CV joints?

 

have you checked the hubs? you could have play in one of them and its letting you know that is time for a new part.

 

I have. 98 blazer that is doing the same thing but when it goes into 2 hi and I drive it makes a clicking noise underneath the truck.. and when I put in 4 hi it goes away..???

 

https://blazerforum.com/forum/tech-articles-diy-29/4wd-front-axle-clicking-when-2hi-goes-away-when-4hi-4lo-41226/
This link will explain what is wrong and how to fix it