I just had this problem too. I ended up using bluedevil stop leak for about 2 weeks. It seemed to stop the leak so far. Others have had good luck with blue devil too. The only problem is you can't leave it in too long. If you do it'll cause the output seals to leak which happened to one of mine. I don't like using stop leaks but I wasn't going to pay the dealership $1000-1500 to fix a $5 seal.
Sorry for the long delay in response. If you want to reseal the entire transfer case, it does need to be removed from the vehicle. However, if you're just looking to replace the actuator seal, maybe the front output or rear output seals, and reseal the case halves, you can split the transfer case without removing the entire thing. Meaning, the front half of the case stays bolted to the transmission while the back half (including the actuator, gears and chain) come out of the vehicle. From here, it's a piece of cake to replace the actuator seal and rear output. The front output is a little harder since the front half of the case is still in the vehicle. It can be a little tricky, but with some patience and understanding of how it all works, it cuts the job time in half, if not more. I can do these in about 2 to 2.5 hours. Any other questions, feel free to ask.
Splitting the case in the vehicle is actually easier than you think. You just have to remove one of the detent balls and the front output yoke, and the entire back half of the transfer case comes right out. The only thing you have to be careful of is not forcing the case while removing it, because if one of those actuator rods comes out, it will ruin the seal for it, which are nearly impossible to find aftermarket and aren't available separately from the actuator from the dealer.
That seems like it would be pretty hard to put the half back together with all the gears and everything aligned but as you said if you know how it goes together and are careful I could see how this would work.
If I remember correctly, the only thing to worry about when going back together is making sure the "clutchpack" is all the way in. The clutchpack actually stays in the front half of the transfer case, and the housing that slides into it is what you have to make sure is all the way in. It's the same as splitting outside of the vehicle though. Either way you split it, you have to watch out for this. Other than that, the gears and stuff are simple and don't make going back together too difficult. The clean up is pretty simple. I use a razor blade to scrape it, then a scotch-brite pad to finish up the surface. The hardest part is finessing the actuator out of the transfer case without one of the rods coming out, but that's not terribly difficult if you stay patient, pay attention and go slowly.
Thanks for the tips. Might have to try this in the future. I much rather have a new seal in there.
[MENTION=152026]Cruiserfanatic[/MENTION] is right about most everything staying on one half of the case. I had a picture of a 5th gen transfer case apart (a coworker was resealing it so I was snapping a picture for reference for people), for the most part aside from the actual Planetary assembly, it all stays on one half of the case. That does sound pretty easy to do with it in the vehicle.
That being said how do you deal with that one annoying needle bearing (or was it a washer? I can't remember it's been a bit) that doesn't want to line-up basically unless you have the case almost vertical? I forget what it's called but I think it's just supporting the shaft end that comes off in the half for the actuator. That thing was always annoying when you're trying to seat the case halves.
Edit:
Here's what I am talking about, that damn thing is so finicky...
The second picture on the right is to show people what stays in the other half that the actuator goes through.
Referenced from: http://www.toyota-4runner.org/4th-gen-t4rs/182074-fixing-transfer-case-shift-motor-leak-within.html
