It's probably the pinion seal, but I find it strange that you're not finding wetness around the seal itself. The pinion nut is installed very tightly with a threadlocking compound, so I wouldn't have expected it to be loosening up anyway. Worn bearings are the more likely culprit here. Good bearings allow the pinion to simply spin in place, but bad bearings allow it to move around and wear out the pinion seal. A fluid leak is often a good indicator that it's time to open up the differential.
At this point, I'd grab the pinion flange and try to wiggle it as SoCal2V suggested. It may indeed be time for an overhaul.
1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver