Wheel get locked up in 4x4

I am not familiar with the model. I wonder does it have differential locks? If it does, that could be the trouble.

In order for an axle to lock there must be some fairly serious trouble with it - something broken inside.

It is impossible to drive a true 4 wheel drive vehicle on a hard surface because the front wheels (in a forward motion) always travel a greater distance than the rear wheels which results in wind-up between the axles.
When such things as crude old army vehicles had to take to the road they would be forced to stop periodically and one wheel would have to be jacked off the ground to relieve the wind-up before they could continue.

Regular road-going 4x4s have a unidirectional clutch fitted in the front drive to allow the front wheels to overrun the rears without wind-up. When diff-locks are engaged a road-going 4x4 becomes a true 4x4 and axle wind-up becomes likely.

From your description I speculate it did not get out of 4-wheel high but instead engaged the diff-locks and it became a true 4x4 and you experienced axle wind-up, something travelling in reverse will relieve.
I further speculate the fault has since corrected itself. It remains to be seen whether it was a one-off glitch...

DOES IT HAVE A DIFF LOCK ? IF SO WHEN ENGAGED IT IS NOT DESIGNED TO GO ROUND CORNERS STRAIGHT LINES ONLY UNLESS GOING VERY SLOW AS ALL FOUR WHEELS ARE MOVING EXACTLY SAME SPEED.

Are all the tires the same size?

are they locking up going straight or turning?

did this start before or after you replaced the rear diff?

if after does the rear diff have the same gear ratio?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

take it back to the dealer and have them fix it properly. It is obvious that it was not road tested before delivery back to you.

ask the dealers what ratio was put in the rear diff
sound like the incorrect ratio against the front differential is used

check out detent limit switches on gear box and transfer case for continuity and open circuit when pressed(on /off switch) these are feed back signals to indicate it has locked into 4x4 or 2x4, if one of these dont work properly system locks up and wont do anything any more. also check front one on diff near actuator, common problem. check actuator is working properly when vacuum applied and removed. can remove this and bench test unit,some oil will be lost when removed for bench test and you can check colar inside diff slides back and forth to lock axle.

there is a central diff(called a TORSEN diff)and yes it has splines on the output shaft.NOW this IS a front wheel drive car that goes into 4wd when traction to the front is compromised(slipping)they are not allowed to call it "quattro"or 4wd as its NOT.jack up the rear and block the front wheels,start the engine and put it into gear,the rear wheels should turn if the central diff is operating correctly.if not the best solution is to get a diagnostic done at a subaru dealer(you can then decide if you are capable of fixing it yourself or are willing for them to fix it)It is not as simple a system as some people would make out as you have electronics to control the front/rear traction.and the diagnostic computer they use is invaluable.AND EXPENSIVE.

1)check that the park brakes are not seized on.
2)check the driveshaft where it connects on the differential.
broken driveshafts and universal joints are common on those.

I have one of these vans you can put it in 4x4 anytime while driving or stopped or taken out of 4x4. the diff lock does not have to be on to have 4x4 .this **** locks in the rear diff,giving you all 4 wheels,this diff **** should be pushed when stopped,and taken out when stopped,it is always taken out before the 4x4 is . ps the 4x4 light on the dash should not be blinking if it is your acuator is stuck it can be freed up for 30.00

What make, model, and year is your truck? Does it have AWD or just 4-wheel drive? What selector knob to you have for 4x4 (gear shifter or electrical switch?) Does your vehicle have limited slip or locking differentials? If you have an electrical selector with locking (limited slip) diffs and AWD, that is normal for it to automatically switch into 4x4 when you loose traction.