whining rear

How you troque the nut on the yoke can change the preload on the pinion bearings and the tooth mesh pattern. That's what caused a whine in my first 7.25 rear.
At about 40 mph ( 276:1 ) it sounded like I was blowing a horn.
A seal doesn't support any weight and really shouldn't begin to leak for no reason.
You can almost bet bearing wear caused the leak. Sorry