depends on which D30 you have. CJ D30s can go as low as 5:38. Hi pinion XJ/YJ/TJ types are limited to 4:88 as far as I know.
Which Locker? It's up to you. If you like having the choice of a selectable locker, The ARB is a fine choice. A Detroit Locker is another great choice, though non-selectable. It really is up to your personal preference and usage.