Did you take a look at the crankshaft where the seal rides? Although, unlikely at 51K miles there cut be a slight groove. I had that problem on a Crown Vic I used to own and fixed it with a repair sleeve.
Another possibility is that the new seal wasn't installed square.
2002 Mustang GT, 2012 F150 SC EcoBoost, 2012 HD FXDWG, 2011 Nissan Juke (wife's)