What's the Squealing Noise Coming from Rear of my 87 5KTQ???

[Bill Rowe]

started happening on chilly mornings.  used to happen only during the 1st
minute of driving.  at first i thought it was the e brakes binding but i
jacked rear wheels off the ground and the wheels rotated smoothly and
there's no play if i try to wiggle the wheels.  now it's happening even
after the car is fully warm and at highway speeds.  seems to happen more on
right hand curves and my son confirmed it's coming from the left rear
section.  bad wheel bearing, bad cv, bad diff, or  low diff oil level (that
would be too easy)?  thanx in advance.
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