Here is what I know... and its strictly from research here. I have no personal experience. (Just got flamed on starquestclub for posting "wrong info" when I was repeating suposedly corect knowlege from the site) Just covering my self. This is also for USDM model years only. EU and JDM models have variations.

So 88-89 starquest lsd's fit the front of a v6 gen1 montero.

83-86 sq lsd's fit a 2.6 montero front. Due to gearing differences, a spacer must be used under the gear to bring it forward for a proper wear pattern. Precision counts, no washer stacking here.

87 was a transitional year for USDM sq. Generally their rear lsd's fits the front housing of a v6 gen1, however the 2.6 montero axles must be used. Again, transitional year, so certain early model 87 sq may have 86 diffs and late runs might just be 88 style. Unlikely, but maybe.

Lastly, Im fairly sure no sq diff fits the rear of a gen1 truck. If you want a gen1 lsd in the rear, find one. Its a factory option.