Yes, you can replace the seal without removing the carrier. If you have an impact gun, remove the pinion nut, tap off the pinion yoke, replace the seal and reinstall everything. Just tighten the pinion nut with the impact and you'll be ok. No need to worry about preload again. Usually, you can hold the pinion with your hand while removing or tightening the nut. Also, while you have the pinion yoke off, be sure to inspect the area where the seal sits on it. If it started leaking that quick it could have a groove on it. If its too deep to polish out, then you should replace the yoke. One other thing, it may not be the seal. Check it very close because it could be coming out around the pinion nut. If thats the case, get a new pinion nut because most of them have a type of sealant on them. That should just about cover it.
JF