I have the exact same issue you described (4wd locking and disabling when slipping on snow producing error code u2023), so I can't wait to hear what you figure out!

 

Update: Removed the OEM Transfer Case/PTU. Installed another unit. (A salvage unit. Insoection of gears and seals etc. showed in in good condition.) Inspected the gears of the OEM unit through the fill port (was unable to do so with it in place due to very limited access).

The gears in the OEM unit seem to be in good condition [:-(]. So my troubleshooting leading to decision to replace it doesn't seem to have gotten to the right answer.

Will update on road testing. May take a few days as there are some other things to do before taking it off the jackstands.

If this replacement did not correct the problem (as seems probable given the gears visual condition) then the next step is the rear axle, differential & torque coupling unit).

 

Road test:
Drove for something over 500 miles with no problems. Have sought to get wheels to spin on gravel. Can not get the front axle to spin without the rear axle so looks like the problem may be fixed. Curious.
The real test will be when I can get to a snow covered surface... Still got some here & will see if I can replicate the symprtoms from last winter,,,

 

Final update: Road test today (4/25). Had several inches of snow this morning up here in Wyoming. Took it out on the road and was able to verify that the rear axle wheels are responding properly to power. When the front axle spun on snow the rear axle spun likewise. They remained in sync with no 4WD warnings.

So the problem back in the beginning was the angle drive to the rear axle.

What I don't get is the visual condition of the gears in the OEM angle drive I removed. The gears looked to be in good shape and the input-output seemed solidly linked when I turned one the other turned against resistance.

At the end of the day the troubleshooting that showed the front axle spinning without power to the rear axle was accurate and replacing the angle drive the correct fix.

This just kicked my butt for 6 months... oh well... been there, done that...

 

You probably just couldn't the load on it that happens under its real use. Was there backlash between the gears?

I've seen a bearing lock up and shear dowel pins but felt smooth as silk when turned by hand.

 

Followup: had alot of rain out here last week... gully washer type rain. Drove up a portion of my driveway that is kind of steep. Wheels spun heavily in the mud. No indication of problems with the rear axle, no warnings about 4wd failure or lockup. If there is a tougher test for this I don't know what it might be.

So in the end, the transefer case I pulled out had to have something like a shear pin failure but with an interference fit that allows it to appear to hold under low load.

If I have it rebuilt (not sure I will) and the shop can ID the fault I'll pass it on.