The rudder lock issue on the DC-3 happens under very unusual circumstances, usually very aggressive forward slips wherein the rudder is hard over, then cannot be moved central again without using differental engine power.
The primary reason is the very deep rudder hinge line.
This is necessary to use aerodynamic assistance for the rudder to counter high pedal forces that would otherwise be necessary.
A demonstration of this was (at one time) required for the type rating, normally done at a reasonable altitude...at least 5000 feet.