Just about to change the fluid in my LSD as well when i noted in the Haynes/Chilton manual they too mentioned an additive. Wasn't mentioned in the FSM though.

I haven't been to a AutoZone/PepBoys yet specifically looking for LSD additive, but in getting the 80/90w gear oil for the front axle and transfer case, I don't recall seeing a LSD additive. Is this a special dealer item? or unique to 4x4 shops?

MI-Mitsu

89 Montero LS 3.0L/Auto
90 MightyMax 2.4L/Man

Parted/gone - parts available!
89 Montero LS 3.0L/Auto [Blue/Blue] (2 of 2)
89 Conquest SHP
88 Conquest SHP
87 Dodge Raider 2.6L/Auto
91 Montero RS 3.0L/Auto

Others: 37 Lincoln