My '86 Cherokee was similarly frustrating to work on. A completely incoherent mix of SAE and metric. My '06 LJ is the same, though slanted more towards metric.
I've always kind of guessed that as components were redesigned/updated, they were redone with metric sizes, since the world is going that way, you know?
In '80, you probably got the first wave of metricization -- might have been Renault's influence as well?