I had the same issue with my rear pinion. Looked at the FSM and seen the special tools required. Almost ordered them too. Then I tore a tendon in one of my fingers leaving most manual work difficult or impossible. So I paid a local tranny shop to change it out for me with the seal i also got from Rock Auto... IIRC, it was a National brand seal.

It's been almost 4 weeks now and no leaks.

As previous poster said, could be dirt/debris either from installation or some junk that was in the diff. I'd do as he suggested, either unbolt and swap, or at least unbolt, remove seal and drain oil and flush out the diff with some sort of cleaner (suggestions from the board?), then really clean that seal mating surface and put it all back in.

My cost was $110. Basically an hour labor (@$70) plus the gear oil to refill. Had I not provided the seal I imagine it would have cost me another $20 or so.

Jim