When fully seated there should not be any splines exposed. If you don't have the axle seated all the way the o-ring on the spline shaft won't seal and that could be your leak. Try tapping the axle in the rest of the way with a hammer and drift, taping on the big section rather than the shaft. That big section should but up right against the jack shaft.
Currently driving: 98 GMC K1500
Project: 89/90 MX-6 GLX hybrid