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Toyota 3rd's aren't weak for the most part. Just the ring and pinion in the TRD elocker 3rd. The other 3rds have been holding togeather through lots of abuse. Even the weak link in the TRD can be taken care of with a quality set of aftermarket ring and pinions.
As a rule you don't want to go as low as 5.71 to keep a strong pinion.
I believe the TRD ring and pinion is the same as a V6 ring and pinion. I have heard that the only weak point is around the spring pads which is why I switched to an 86 rear end. I could be wrong however.
