They are wore out. Time to switch to manual hubs . Bronco graveyard or maybe even on here in the classifieds. New ones are outrageous. I've got some manual hubs, just dont have the conversion kit. New auto hubs will be over 300. They fail cause they are 20+ years old. As long as you stay with the D28, switch ro the manual hubs.
Id take 40 for my manual hubs. I've got a set of autos, but would not recommend using used auto hubs. You would need another 30 for the conversion kit.