Another update.... good and bad.
Good news is I've run 6 months and something like 15k miles on this setup including going across the country light and coming back pulling a gooseneck trailer and a parts truck at 23k gross. No issues and the dual range was a lifesaver with that much weight. I used 4th under quite a lot on all but the really steep grades.
That load was from CT to southern CA and then I turned around and took my other trailer and a different truck to norcal (~18k gross). The original plan was to also pull a car behind the flatbed on a dolly but that idea lasted about a quarter mile.... made it up to about 35 mph on a slight downhill and the trailer takes to whipping all over the road.... yep, that idea isn't going to work.
Round about 400 miles into that trip the entire transfer case & dual range assembly tries to fall off the truck.
Didn't have any real warning, just a sudden harsh vibration and loud thumping noise. In retrospect there was some vibration prior, but it was fairly minimal and easy to dismiss as road vibration. Couldn't figure out what was wrong at first till I crawled under and started playing with things - the rear output flange bolt on the dual range was loose and the rear of the dual range wiggled up and down a good 3/4" when pushed.
In total I found the following:
-The bolt and washer retaining the rear output yoke to the dual range shaft had come out completely.
-5 of the 6 transfer case to trans bolts were stripped. The one that was not stripped was one of the bottom 2. The other bottom bolt may have been previously stripped (IIRC one of the 6 was partially stripped, not sure though and can't recall for sure which)
-Both of the trans mount to trans bolts were missing. One or both of these may have been previously partially stripped.
Thankfully nothing serious was damaged, and between the one bolt and some creative ratchet strap use I got it secure enough to get to the gf's house in Alturas (destination for the truck on the trailer) and then was able to stick studs in the holes and get enough undamaged thread to get to the ultimate destination in WA. Long range fix is heli coil or replace the t case.
The big question that I'm still contemplating is what exactly the root cause was. The one undamaged bolt is at the bottom and both the middle as well as the one top bolt I took out had a solid 1/2" of thread pulled out of the case - ie the thread roots sheared and the thread was completely pulled out as a unit. This suggests to me that the case broke off downward - in other words it was not pushed up by the driveshaft bottoming out. I suspect the yoke bolt came loose first. However, even without the bolt it still wouldn't have been able to move that much. Any serious movement would also likely have caused more noticeable vibration.
I did initially have concerns about the extra weight added to the rear of the t-case and installed a spring/turnbuckle setup to try and add some support at the rear. These may have come loose over time.
I have found that shifting the dual range is hard to do smoothly, and it tends to cause some fairly severe torsional shock if you're not careful. I suspect these shocks are what caused the yoke bolt to loosen. I am also in retrospect unsure if I used loctite on the bolt.
I suspect the combination on the unsupported weight and vibration from the loose yoke is what caused it to fail. Interestingly I was on a fairly smooth flat section of road when it finally let go. The way the bolts stripped would require quite a lot of force, and I can't think of the way they would have progressively stripped. Maybe fatigue? It's odd to me that they all failed the same way.
It's nice that the bolt hole proved to be the weak link and not anything harder to fix. I would have expected the bolts where the dual range adapter meets the t-case to have been the weak spot since there's only 4 and they are only designed to support the stock extension housing.
The great unknown is of course if there's any other damage. There's nothing apparent yet, but it can't have been good for any of the trans/t-case parts.