First I'm not familiar with your trans axle as in having one apart.
I looked @ parts tree diagram and understand the system.
A ray of hope here:
I am thinking you just might get lucky in as much as your description of the long trailer trips it underwent, rocking back/forth.
It is possible that the notorious loose bolts that hold the diff/ lock assy together, decided to back out during the trips and are what is locking it up.
Hopefully as it was not being operated when it happened as your test drive was successful, the damage will be minimal.
Now back to the others who have BTDT.
