Am I glad that I fixed my 4 wheel drive! I've needed it a few times this month. The unfortunate part of this update is the fact that I now have a very defined "clunk" in the front differential. Of course, this is just speculation at this point simply by where and when the noise occurs.
The Blazer has a weak point in the transfer case (a snap ring that breaks causing a "grinding" noise), so that was my first thought, but quickly ruled out -- it only does it in 4 wheel drive for one. If it were the snap ring, it would grind in 2 wheel drive and get better in 4 wheel, not to mention the noise would be coming from the rear seat floor (where the transfer case is) and not the front end (which is where the front differential is).
Anyway, I have to do a little more diagnostics with the guys over at the garage on a drier day to figure this out. I'll keep you posted!
Wow! I thought I had updated this post to describe the problem as it was found by the guys at my tranny shop.
I really should get some proper terminology for the parts -- anyway, as it was described to me, there is a set of "forks" in the transfer case that control the movement of a gear (gears?) for the various modes (2Hi, 4Lo, 4Hi). The forks have plastic (delron? nylon?) bushings that quickly wear out -- this is an inherent problem with the transfer case of my particular year of Blazer -- and cause the gears to not fully seat in their proper positions. This means that when you try to switch to 4Hi, it may not go in right away or, for that matter, when shifting to 2Hi it will seem to stay in 4Hi for a period of time, then switch to 2Hi with a "bang". It may also kick in and out as you drive down the road, as mine did.
Again, I should really find out some more information on this problem. As I have said, I have a tranny shop in the family, so it was repaired and put "on tab", so I can't even tell you how much it cost to repair, but I will find out.