Lot's of variables here. Do you run rubber motor mounts or solid? Motor plate? Mid plate? Poly trans mount? If you run solid motor mounts or a motor plate and you use a poly trans mount there isn't enough cushion in that hard trans mount to give and the weakest point of the extension housing is right behind the webbing, probably where yours broke.

An out of balance driveshaft wouldn't have much of anything to do with it. The bushing inside the extension housing rides on the yoke which is attached to the output shaft.