Driveline "clunk" in 4wd

[Boyd Who]

I searched the site a bit but couldn't find anyone with this problem.

I was out in the X today on a snowpacked highway. I pulled over and stopped, then put the truck in 4wd to see how it handled. Everything seemed fine, except that every few hundred yards I would hear a clunk, almost like some ice falling off the truck and being run over. I was going straight, and running about 45-50 mph. It did this a number of times before I put it back to 2wd.

Any ideas?

 

[MrShorty]

Idea 1: Auto hubs slipping. The auto hubs are fairly prone to various problems.
Idea 2: The same thing used to happen last winter to my BII, because I had mismatched tires front to back. They were all the same "size", but different manufacturer, style, etc. This was enough to create enough difference in size back to front for the 4WD, when engaged, to bind up (even on snow covered or muddy roads). I also had trouble getting it back into 2WD due to the binding.