Is it coming out of the seal area or in front of it? When I do the final assembly I use a little rtv on the splines of the yoke. Not much, but a little. I've had drips from oil that found it's way through the space between the splines before. Not usually a fast leak. But a leak. I've also seen the seal leak between the case and steel jacket of the seal when the case was damaged (scratched) removing the old seal. A little rtv on a new seal can stop that too.

Oh yeah, one other thing... make sure the vent for the axle is not plugged...