John... in nearly 40 years of doing this, I've only replaced about 4
transfer case chains... all of these were on units that saw a LOT of 4WD
heavy slogging.... Most concerns involve shift forks and/or mode
synchros....

I'm probably the original "doubting Thomas"... I like to have a very good
reason for anything that I do to an automobile...

Looking back through posts on this NG and other venues, I see a few
references to debris on the transfer case speed sensors.... I recall some
reports (and, IIRC a TSB) about transfer case fluid contamination. Neither
of these really seem to fit your circumstance though they are worth
exploring - as in cheap to do.

Unfortunately, I'm not close enough to the concern to be of much more help
at this time... you sound to have a lot of money invested in this concern
with no resolution and I am loathe to recommend a course of action that I
wouldn't normally take on a customers car....