I had a similar issue with my 98 Overland. 8.3L cummins, Allison 6-spd. Cats-eye error, no shift, dead where it sat.
Did all the tests, made the calls, replaced the TCM with a borrowed spare, almost gave up.
Although all the wires passed the tests, I ran new power and ground wires from the battery directly to the VIM, which powers the TCM. Spliced them in directly at the VIM.
All the issue I was having disappeared.
Worth a try on yours.
Mike and Carla
1998 42' Overland Larado Diesel Pusher
On the road - here and there - Full Timing