While it's comforting knowing that all KJs do that I still find it hard to believe that's how differentials should work. That means it should bind in 2wd to since front and rear differentials would also be compensating for speed difference in U turn. It does not bind in 2wd, though.
"Torque" is why you get binding in tight,slow turns in a KJ with the 242 t-case.
When torque is applied to the differential it naturally wants both axles(or front/rear drive shafts in the t-case) at the same speed,the spider gears have alot of force being applied to them that takes a second or two to overcome and even then has alot of resistance to letting the axles(or driveshafts) spin at different speeds.
Without torque being applied this will not happen and why you don't feel it while in 2wd since no power("torque") is being sent to the front diff and the center diff is not being used.The rear diff is being "dragged" and the wheel are not "steering" so that slight binding is not felt.
Plus the 242 t-case is setup to bias the front/rear torque split(48/52 if I remeber correctly) since the KJ uses mis-matched diffs(D30A front and 8.25 rear) which do not have the exact same gear ratio,the front diff will spin slightly faster then the rear by design.You don't want the rear end to spin faster which would make it uncontrollable trying to drive in a straight line in 4wd.
The 242 t-case is almost a 40 year old system so it's pretty crude compared to the weak crap they have today.