I believe the FSM lists a light coating of engine oil for the auto hubs. You may want to download the FSM. Give a copy to your buddy if he's going to be working on your truck.

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

Mitsu # 7 - 02 6G74 Montero Limited