Check the diff breathers. If they're not working, it can build up too much pressure in the diff as they heat up, and blow the 90w past the seals. If they're new, and you didn't damage them on the install, they should survive the gear oil blow by. Might want to check the knuckles and make sure you don't have wheel bearing grease soup (mixed with 90w). Could cause an early death in there.
While your at it, if you plan on wheeling it where there is a chance of water crossing, save yourself alot of headaches and put extended breathers on it. The redneck way is to pop the spring loaded cap off, get some fuel line and clamp it on, then run it up into the engine bay. There are some metric adaptors you can buy, which make this more professional, but either way works well. Some like to put a fuel filter on the end of the hose to keep those pesky mud-wasps from building a nest in there (ask me how I know). Not sure on the fj80, but on the 4Runner, I was able to run the hose on the rear housing up into the tail-light assembly, keeping it out of harms way.
Erik
