See Issue #24
Joe D did a write up on the diffs. He stated that a leaking pinion seal is (in his experience) usually caused by bad bearings.
The good news is that you can disassemble the diff and replace the bearings without going through the set up on the 80 (I'm not sure about the others. )
While you are in there, replace the carrier bearings too. Also, check the hub bearings. These things do wear out.
You are torquing the nut back down to 450 ft-lbs, right?