If it is related to the transmission , and you were not made aware of any fluid leaks it is possible that during the axle shaft replacement there was enough fluid loss from the transmission when the axle was out and it was not filled back up after the job was done.
I checked the fluid level under the transmission filler cap and it was full to the cap.
You removed the bolt on the front side of the transmission below the vehicle to check the fluid level?
No. There wasn't any grinding or any other warning sounds/noises from the transmission to indicate a seizure due to insufficient fluid levels. It was a sudden pop as if a component was dislodged.
The next step would be to have the transmission removed and disassembled to be able to inspect the drive gears , lock rings and bearings inside to determine where the failure occured.