Thanks guys, however I already knew about that trick from the evader website (I belive it's yours cjg? it has the same pic than the one in your avatar, if that's the case grats it's a nice website where I got a bunch of info when I started with my evader).
Anyway, my problem is not about dirt/dust entering through there, the washer, thrust balls and components around are pretty clean past the initial nut, it's about it entering in the space between the outdrive and the gearbox, it's there that all my dust/dirt enters and goes straight into those giant outdrive bearings (the 12x18mm.).
The rest of the box and bearings have no problem and stay clean.
I've tried duneslider's idea of grease before butting in the bearing, but it didn't help tremendously, maybe because the grease I used wasn't especially thick.
Yesterday night I rebuild/recleaned my diff and I found a sort of ghetto solution that should work but I'm still open to ideas. I made two little cylinders (in carton) that I put around the outdrive while letting some space to spin freely without touching (cyclinders must be 8mm or so bigger in diameter than the outdrives). Then I taped the cylinders to the gearbox so there is no space open between the cylinders and the box. Hopefull this will prevent dirt from entering from the outdrive/gearbox space since now the dust would have to travel through the whole carton cylinder laterally before getting into the crack.
If anyone has some more suggestions on a more permanent fix, let me know.