There have been a ton of postings on transfer case heat. Many ask about a cooler setup or other ideas to reduce the heat...basically there are 4 ways you can go:
1. Have your NP200 rebuilt, cost around $250-500, and in most cases, the heat problem will be gone.
2. Find and mount a pump that will pump gear lube, find and mount a cooler, possibly one with a fan on it so as to keep the lines shorter...don't let stuff hang below the frame rail unless you plan to disconnect it everytime you go wheelin, if so put in quick disconnects and shut off cost will be several hundred bucks...cheapest pump anyone has found is over $200 itself.
3. Find and mount a cooler very close to the tcase, preferably with an integral fan...put shut off valves and quick disconnects cause at least the bottom one is gonna stick down to far...plumb it with one line to the cooler from the tcase fill plug and one from the drain plug. Then hope that by convection alone, you will have enough flow to make it work good....also remember, there isn't a ton of room to make nice short straight lines, but it may be possible...cost, probably in the $200 or more area and a lot of one seems to know if it will work effectively as a cooler especially at high speed/high temps in the M715 environment.
4. Get a NP205 for around $150-$200 and with some angle iron, adapt the stock mounts to fit the 205 and call it a day...not only wil the heat problem be gone, but parts are more widely available, a 3:1 low range kit is available, and it is just as strong as the NP 200.