How you troque the nut on the yoke can change the preload on the pinion bearings and the tooth mesh pattern...
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
x2. I dont think if it comes and goes that it's dangerous or enough to destroy the rear. But, by unbolting the pinion nut you did "take it apart" and happened to hit the crutial fastener. On some rears removing the pinion and yoke do little. On others, it might as well come all apart. What did you torque the pinion nut to? did you run a pattern before you installed it? Backlash is important, but I'd rather see a pattern.