2000 Blazer LT 4x4. We got almost 3 feet of snow this past weekend, needless to say I need to use my 4x4. When I am in 4x4 or Auto 4 I have a loud knock from the left front wheel area, but only in 4x4. I confirmed that my front diff is full of fluid and 4x4 does work. I looked underneath and nothing appears loose or broken. I did notice yesterday that it will not go into 4 Lo. Any ideas what I should check?

 

Im thinking a bad CV joint. Could also be slide collar that engages right side axle is stripped or not fully engaging.

 

The axle is only a few months old. So I guess it could be the slide collar. Oddly enough it didn't do it yesterday.