at this point, you could try the stop leak stuff,lucas, stp, or half a doz others...but your best bet is R&R the gearbox and have done with it. You might be getting some flex in the gear box from your frame rail. A good gear box brace might be in your future.
'Betty' had a similar leak as yours and as it turned out I wanted to upgrade her lower end steering components as I'd come into some money after tax season. Got a heavy duty steering box mount and heavy duty pitman arm from Sam's Offroad (check their site there's some interesting reviews) Took the works to a local shop as I'm sure the alignment was off as well. After these parts went in and a rebuilt Saginaw box this thing steers easily with just a finger, and track is so straight I can do the "look Mom, no hands" (although I never do).