can be a few things causing the seal to leak or fail.
the problem is you would have to open things up and see.
you want to know where it is leaking from exactly.
you could have a damaged seal that occurred during installation, a ripped seal, a grooved axle, the seal could have been installed crooked also or was not fully seated, maybe the axle case has an imperfection causing a leak as well.
now changing the seal is actually quite easy i never did it before but 3 years ago i rebuilt my rear axle and i changed the seals and bearings, and inspected the area. i used a seal driver and made sure it was sealed correctly before installing the axles with new back plate seals as well.
basically jack up the side that is leaking.
pull the drum,
pop the 4 bolts holding the backing plate to the axle remove them. T
Turn the drum over,
put it back on the axle and put 2 or 3 lug nuts back on looesly.
push the drum in on the studs then pull, you just made the drum into a slide hammer to pop the axle loose from the diff splines.
undo the lug nuts, remove the drum.
carefully slide the axle out a little. put 2 of the bolts you took off from the backing plate back through the axle and the brake backing plate, this will support the drum assembly. Keep the axle level and pull it out.
inspect the bearing seal area, run your finger over the wear point it should feel smooth and no grooves if you can catch a nail in a worn groove the axle is done replace it.
inspect the splines on the end. if the splines are twisted, junk the axle get a new one.
you pass test one and test 2 move on to the seal itself.
before you pull it look at it, any rips, deformation in the outside ring, from bad install, put your finger in it and feel the back side, can you feel the spring on the back side, it is like a slinky and it holds the I.D. of the seal against the axle. A bad install can damage the spring and it leaks and leaks.
if you find a problem great! if not look at the seal is is straight is there a imperfection on the axle case... well the stupid thing is leaking anyway, rip it off, using a seal remover or flat head screw driver its like poping the cap on a glass bottle of beer basically.
seal out inspect it again, get size information from the outer ring. remember which way they seal is installed, take a photo
reference:
order new seal.
use a seal driver and install the new seal hammer it straight and make sure it seats on the axle lip inside.use light taps and a rubber hammer, you don't beat the thing in just lightly bring it home so it isn't marred or damaged.
you don't need sealant the seal has a red sealant like paint on the outside mating surface this forms a seal when hammered into the axle.
with the seal installed and straight. get diff fluid. wet the seal with fluid. next wet the axle shaft with diff fluid so as you slide it into the axle it does not rip the rubber seal.
now keep the axle level and slowly carefully slide into the diff housing, try not to rest the axle on the seal as you push it into the axle case. before you seat the axle pull the bolts holding the brake drum up and support the drum and the axle, try to keep them off each other. Twist into the diff splines and make sure you are fully seated, put a bolt in to hold the drum backing plate and go take a shower hahahahahahah
torque the axle backing plate bolts to spec and then you may want to jack the car up in neutral and spin the rear axle to make sure you don't feel or hear anything funny.
now if you have a ford 9" you can use the fill hole on the drivers side. for an 8" you may of needed to fill the case through the axle shaft before install(i did that once)
but for the 9" you fill up the case till it comes out the fill hole itself if you have a traction lock remember to premix the amount of friction modification lube you need then fill the oil up.
cap the fill hole and then reinstall the drum and wheel and go for a spin and then inspect the area for leaks for a few days.
now lets say it still leaks but you know you did't damage the seal... then i recommend taking the seal out again, and install seal #3 only this time when you hammer the seal in do not fully seat the seal let it stay off the axle lip like 2-3MM and try again. sometimes an axle wears with a tapper that you cannot fell and moving the seal out a little guarantees a fresh area of surface contact. it happens.
i never did it before busted my cherry on mine and have been good for 3 years so far.
The reason you jack up one side when you pull and install the axle is so the diff fluid that is still inside doesn't shoot out on you and the garage floor when the axle shaft comes out.
No big deal, just replaced the line with rubber line until I can bend up a piece of hard line.
