nothing
There's a big difference. The FJ80 hi-pinion diff is designed to be properly lubricated in it's normal operating position, and drives on the running side of the gear set, rather than the coast. If it's placed in the rear axle, it is then operating on the weaker coast side of the gear set and is improperly lubricated as well.
The reason one can get by using a rear axle's low pinion diff up front ( ala 79-85 Toyota single axle) is because the low pinion doesn't experience a lube problem - it's simply low enough to be bathed in lube oil. Secondly, although it is running on the weaker coast side of the gear set employed up front - normal (<10% of the time) 4x4 usage doesn't stress it enough to make a whole huge impact on its life.
When a hi pinion diff is placed in the rear axle, it's used 100% of the time and won't give you a normal operating life (usage).
