This is a great post, and I just wanted to update it with some part numbers and pictures for the drain plug and gasket.
Drain Plug: 90341-18021, torque to 27 ft-lbs.
Gasket: 90430-A0003 (previously 90430-18008)
The drain plug has a small magnet on the inside. It is important NOT to substitute for this drain plug the other, longer magnet drain plug (for example, the one on the rear differential) because you will run into clearance issues inside the case. You can reach your finger into the case after draining it, and you will see what I mean -- there are components within about 1 and 2/3". Use only the short plug.
Below is a picture of the new and old gaskets. These are aluminum alloy gaskets, and are the same gaskets as used on the automatic transmission drain plug. It is always tempting to re-use gaskets. But the picture shows why that isn't always such a good idea:
The picture shows that the aluminum alloy is well past yield and plastically deformed, so for this material we know its into strain hardening (cold worked). The plastic (inelastic, permanent) deformation is normal for a used metallic gasket, because it is by deforming that they work to seal mating surfaces against fluid leak, limit clamping force, and restrict thread engagement when torqued properly. The problem in re-using such a gasket is that the strain-hardened material will not deform as easily as a non-strain-hardened (ie, new) material, so it will not seal fluid leaks as well and the clamping force implicit in the original torque spec will not be appropriate for the stronger material (for connections with gaskets, the crushing resistance of the gasket increases the torque required to drive the bolt once the gasket surfaces are compressed).
Maybe its not such a big deal for a transfer case plug, but its something to keep in mind when inspecting any gasket you wish to re-use. But even for a transfer case, re-using a gasket too many times is likely to lead to leaks, over-torquing, stripped threads, or all of the above.
The over-torquing is perhaps the most common/annoying of these problems for this particular bolt (and all the other hex-key bolts on Toyotas), because it has a hex key (Allen) head. If you encounter an over-torqued drain plug, you will likely not be able to break it free using the 10-mm hex head -- in this case, try a Torx T-55 instead, and plan to replace the plug. Also be sure to hit that plug with PB Blaster before you try to open it, and wipe the PB Blaster off the head socket before you insert the Torx socket. The Torx head will help you get the plug out, but it will likely deform the head. Below is a picture of a hex key drain plug which required a T-55 for removal, and you can see the damage. You might get one more use from a drain plug in this condition, but in my opinion, a plug like that is worth replacing to save the threat that next time it won't be so "easy".