I think I found my problem. Thank the heavens it was not the transfer case in the end. This UNI joint has seen better days.
That cap on the left has smashed to pieces just as I pulled into repco to get a 8mm socket I didn't have. The one on the right was just full of powder where the roller bearings should have been. Now down to two spare unis for the rest of the trip. 30 minutes to fix on the side of the road though and not something major like I was fearing.