Front axle shaft brakes at u-joint yoke

1983
Chevrolet
C-20 4x4
6.2 Diesel
192000 miles

Replaced inner and outer axle shafts installed new u-joint on left front drive shaft. Old axles shaft broke the ears where u-joint attaches . After about 50 miles inner axle broke the ears off. The only change made to the truck was to change out automatic locking hubs to mechanical .What could be causing axle yoke breakage ?
I've never seen this happen without having a bad u-joint 'destroy' the yoke. Metal fatigue, maybe? You aren't driving in 4WD on bare pavement, right? The front differential isn't 'locking' up? I can't see how the hubs could cause this problem, unless maybe, one side is not unlocking, but still can't see how it'd break yokes. (and that's no yoke).