Axle also ratio depends on ELSD or not. You can find axle radios here:
Jeep Grand Cherokee WK2 - Maintenance information and schedules
5.7s have 3:45, 6.4s have 3.70. Dunno if they are swappable, don't think anyone sells the gears without the housing anyway (about $1500 each end). You may be able to get gears from a salvage yard.
Part of the reason the 6.4 and 5.7 use a different ratio is they also have a different transmission with different ratios.
Dunno about the rear, but you do have to pull the entire front diff assembly out of the chassis to split it and work on gearing, a 2-3 hour job each way.