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Not all ratios can be matched perfectly front and rear, depending on manufacturer.
That 2-3% was the general rule of thumb for 4wd, and probably still holds true. It was common for a 4.11 rear and a 4.09 front to be matched from the factory.I think the general rule of thumb is within 2-3% and you are good.
On an AWD vehicle this is why it is important to run the exact same diameter tires front and rear and at the same tire pressure.
Back in the ZJ days people used to complain about how short lived the viscous couple in the transfer case (NV249) was - I'm guessing a lot of premature failures were due to mismatched tires and pressures. If the tires are spinning at different speeds the difference has to be absorbed somewhere - in the transfer case in the form of heat and wear as well as tire wear.
In an XJ with AWD mode (NV242 transfer case) this was a non-issue since the centre diff is an open, mechanical diff. In the NV242 configuration, a mismatch between tires and pressures can make the transfer case difficult to get into or out of AWD mode which can lead to mechanical issues. I've experienced it in my XJ but haven't broken anything to date (~280,000kms).