I bought a 1998 Blazer as a first vehicle for my son. It has 103,XXX miles on it. After we brought it home, I noticed that it was making noise from the right front and ended up replacing the transfer case vacuum switch, which solved the problem. Now that it has gotten cold, when I switch it to 4hi, I am hearing a rotation type grinding noise. It sounds like it is coming from the right front. When I put it back to 2hi, the noise goes away and it drives as it should. A shop I took it to said it is the front differential, and said there were some metal shavings in the gear oil when they removed the drain plug. Not that I don't believe them, but I have noticed that the noise gets much worse when it is cold. At 10 degrees outside, the 4WD made a loud howl until I disengaged it. At 25 degrees, I hear the rotation noise. Above 30 degrees, I barely hear anything at all. My first thought was the driveshaft CV joint, as there is a bit of up and down play, and any water/dried grease in there may cause more noise as it gets colder. I can't justify $1700 for a front differential and hoping someone has had this problem and found it to be something other than a new diff. Thanks for any help!

 

Based on your description of the problem, I am going to say that it is still a problem with the front axle engagement. I created a thread in the Tech Article (DIY) section describing the reverse of your problem - 4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO. Read through that thread. You may not have to replace the entire axle, just the pieces that handle the engagement of the front axle inside the passenger side extension housing as shown in that thread.

 

Thanks! I read the thread and I will be testing all of it tomorrow. I forgot to mention that I did check the accuator, and it looks like it is working as it should. Thanks again for your reply!