I know we're splitting hairs but the shudder you feel in a real plane is the stall shudder in game. What is happening with the reference to the P-51H is that you start to get small pockets of separated airflow over the wing before the entire wing stalls. So while you can have small burbles of separated airflow, the manual will not call it a "stall" until a point where the wing is not generating enough lift to support the planes weight.
As Hitech has stated, once any part of the wing is stalled, that's what triggers the stall shudder in game. So unless Hitech has changed that modeling (which I doubt), there should be no shaking until you get into airflow separation.