Do you have the 89 Parts manual? If not, you can download it from mitsubishilinks.

I'm guessing you are looking for MB092440. It's available for $2.50 from mitsubishipartswarehouse.com. It looks like it was used on many models, all the way thru 2004. You shouldn't have any trouble finding it.

Mitsu # 6 - 98 6G72 Montero Sport, Locking Rear Diff, Aisin Manual Hubs, 31x10.5

Mitsu # 7 - 02 6G74 Montero Limited