Guys, I don't know hardly anything about automatics. But IF we were talking about cracked clutch discs at low mileage, I can offer something.
I'm sorry, I don't know how the truck is built and some "details" but here is the one comment.
Dowel sleeves.
Crankshaft to torque convertor/inputshaft mis-alignment, both concentric and angular.
Repeated transmission installations MAY deteriorate the mating holes or the dowel sleeves or missing dowel sleeves. and you can loose some of the precision alignment, misalignment = flexing = starts cracks, cracks don't heal overnight.
Again, I just saw the thread and claim no expertise on A/T's, but I have seen the problem on handshakers. Just my $. 00001 worth.