pull the driveline and look at the slip yoke, if there is a groove worn into it, that is probably the cause.
the other thing to look for is the clearance between the slip yoke and the tail housing, if it is too much it too will take out a seal.

cure for the first problem is to replace the yoke.

cure for the second problem is to replace the tail housing bushing.

On my car, if i grab the driveshaft yolk and wiggle it around, it moves. Like the splines are worn or the output shaft is loose.
Ever since i had trans rebuilt, for other reasons, the output shaft is not as sloppy and it doesn't seem to leak as much.
I don't know, that's what i see on my car.

i have the same problem but i found that the yoke needs metal spraying cause the seal cant seal it anymore cause all the metal is gone

Sounds to me like someones replaced it before, and not been careful about how they used a screwdriver to remove the leaky seal. Even a small scratch will quickly chew out a seal. Also make sure that you aren't putting too much oil in the box either.

Check the seal for any unusual wear. If it continues to leak, use a thicker grade gear oil.

If the surface on the output shaft is scratched, use some 1200 grit wet and dry on the surface. Also make sure that you are using the right seal part. Maybe they are selling you one that's close...but not quite the correct size.

Chris

Hi does anybody know how to cure a oil leak on the tailpiece of a stock 5speed gearbox,i have replaced the seal 2times now and it lasts for about a week then it starts leaking again.