The rear locker is standard equipment for this year. It's one of the reasons I chose this model. I'm fairly certain the ratios are correct, as this is a new issue for me. It has never done this binding up before when I push the button.
You can enter your VIN into this Toyota website and see what equipment it had when new:
Vehicle Specification | Toyota Owners
I don't think any models had standard rear lockers. But I could be wrong. I know they're more common on Limited models, but still far from common. And pretty rare on SR5 models.
My '99 is a factory manual locker, and it came with the #RARE 4.10 locker diff in the rear. Manual transmissions only came on 'lesser' models (mine is a Highlander package SR5), and lockers were a bit more rare on lesser models as well (where people were presumably watching that total price and not just checking option boxes with abandon), so it's a combination of two rare options on the same car.
My diff started making a whirring noise at around 215K miles, and armed with 'enough knowledge to be dangerous' I found another locking diff and swapped it in, since it was from the far more common automatic lockers, it was a 4.30. And I just assumed (bad idea) that since 'all lockers are 4.30' that mine was a 4.30 as well. Well... apparently 98% of all lockers are 4.30.
Anyhow, I did drive it for a couple of months with the 4.10 in front and the 4.30 in rear. And it really wasn't as bad as you'd think. It's a pretty small difference, and since mine is the SR5/manual trans, it doesn't have the AWD mode - so the only time you use 4WD is in a low traction situation, where the small difference in gearing wasn't enough to make an issue or make it obvious something was off. I did certainly get a bit more tire squawking than normal out in Utah going over bits of sandstone between the sand (like at Elephant Hill) - but barely any more than my wife's manual '96 was doing -with matching diffs. And I'd almost want to say that in some situations it actually worked BETTER - when turning tightly in 4WD the front wheels tend to push wide a bit - but having them turning slightly faster than the rears meant that it would more readily turn in in slippery conditions.
Still, I did notice that I could *never* get it back into 2WD without backing up (just a foot or so), going forward in an S pattern didn't do it. So I eventually put it in 4WD, jacked up one side, and rotated the wheels around and around... and saw them going out of 'synch'. D'oh! Got a 4.30 front diff, swapped that in, it all matches now.
