As much as I am familiar with driving a lot of heavy trucks I am not familiar with modifying them , , at least you know you have an Eaton diff , the axles will be floaters so you can slide the axles out without removing the wheels at all , but you want to undo the lock nuts & remove the hubs , I am not sure what would fit the Eaton diff but it should be possible to swap some hub onto the diff housing .
As for disc brakes they are really a new thing almost every heavy truck uses drum which is retarded IMO , possibly a whole diff with air ride & disc brakes maybe the easiest swap
