well, someone has to be the village idiot! I do everything
on all my vehicles including the engine swap on my HL. When I did that job, the transfer case/diff and output seal were basically bone dry/slight seepage at best. Not enough to tear into at that point. When I pulled the driveshaft with the engine on the ground, it leaked some diff fluid (I didn't bother draining first) so I knew it was low when I got everything back together.
The engine swap went almost perfectly but I noticed it was immediately leaking fluid from the output seal. No big deal, I probably damaged it with all the moving around and it was likely original anyway. I installed a new one and that immediately leaked as well. Damn, I must have done something wrong so I installed another one and this time drove the seal in a hair more so it would ride on different part of the driveshaft yoke. It immediately leaked and so I just lived with the leak for over year and have been topping it off it off at every oil change. There's gear oil everywhere, it's a mess, so I decided something had to give. I made an appt at my local Toyota dealership to see what they thought.
They sent me a video link with the Tech walking around the car, everything looks good but he says the transfer case is overfilled. Huh? He then points to the fill plug I was using which is the wrong one!! There are three
plugs, the one up high in the center is only used to set the fill at the factory. That's the one I've been using all this time. They cleaned it up, got it to the correct level and it doesn't seem to leak a drop. They could have easily taken advantage of the issue and it only cost me $150.
Now I pieced together what happened. When I first got the car, I changed the front diff fluid and did it correctly. I don't even know if I noticed the upper plug. With the engine out, it's right in your face so I didn't give it a thought and just kept going back to it over and over. I can't be the ONLY one who's done this so I'm hoping this will help someone else at some point.
I still never got my eyes on the vent