Some simple seal info:
Make sure the vent is not plugged
Use a double lip seal, as the flange is usually worn with a ring depression from the oem single lip seal. This gets you to a new surface with a least one lip.
Make sure the axles are not hanging when checking fluid level.
And if the rear is good and the turning torque is good, you can index the pinion nut to go back exactly as removed.
You can also use a string and a fish pull scale to make a before/after comparison.
Snap-on inch/lbs torque-o-meter if best tool for this job.
And if the seal was leaking for somewtime, look to see if the pivot pin is still centered in the rubber hanger mount. Leaking seals rot the rubber buffer interior of the mount and the clue is an off centered pivot pin in the mount.
This mount can be changed in the car. need be................