Latest update: I just tested both front IWEs under vacuum. They both held 16" with no issues, including when rotating the wheels.
I also tested vacuum level from the system at each IWE, which came in at 26". There was no variation when cranking the wheel either.
Since everything seems to be locking and unlocking as expected, but it will still go without a hum for a short distance after going full left lock, I can't help but wonder if it's the wheel bearing getting shifted a bit and re-seating to the noisy position after that distance (which is weirdly consistent, by the way). There was no discernible play in the bearing, though.
One thing I hadn't mentioned is that this vehicle spends a large amount of time on the gravel. I didn't see anything, but maybe there's a stone lodged somewhere? Kind of grasping at straws...