I've been meaning to post a brief update here ... This one will be very light on pictures.
While returning from the ONSC Christmas get together, I started noticing a pretty substantial drive line vibe off throttle at ~90kmh or better. Below 70, all was good. I got off my a-- and got under the rig and started shaking parts, expecting to find the rear output of the transfer or the pinion to have a bit of radial runout ... nothing. Then I grabbed the drive shaft and shook it and noticed the transfer case companion flange "wobbled" on the output shaft.
I pulled the drive shaft and was greeted with a pretty large cascade of oil out of the flange where the nut is. The nut was fairly shiny indicating a new part. When I checked for play the output shaft was good, but the flange rocked on the shaft. The nut was staked correctly, but loose.

I buttoned things back up and figured I would check the oil levels. I found that I could not remove the fill plugs as they appeared to have been put on with an impact wrench ..

A quick call to
@beno got a few parts coming to me to R&R the transfer output seals. While I have no doubt I could have R&R'd the seals myself, I have never been into a split case and would have no point of reference with respect to what the shafts / flanges should look like. I took the truck to a local shop I trusted and let them handle it. A few $$$ later, and the consensus was that the rear output flange retaining nut had been installed finger tight then staked, but that there was not any excessive wear on the shaft or flange

. They also agreed with my suspicion as to how the fill and drain plugs were installed since three of the four had to be cold chisled out.

Way longer than I intended, but the truck runs much smoother now and I dodged a bullet on this. It appears I caught the issue before any real damage was done ...