I don't believe they offered 31s until model year '89 on the trucks and '90 on the 4runners. As for having a 2 pinion on a V-6 truck, that was not very common, but being as you have a very early V-6 truck, that might explain it. The 4 pinion diff was first offered on the turbo 22RT-E models, but may not have been commonly offered on the V-6 models until '89 and later.

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