i have 489 case open... how/what do i need to make a sure-grip?

You can buy a Powertrax Lock Rite and convert what you have to a similar unit to the Detroit Locker. It will operate in much the same way. It will not be a posi. It will lock the axles together under power and release them to go around a turn. They actually make an open carrier much stronger because they eliminate the spider gears. They are pretty nice and will handle around 400 HP possibly a bit more.