Transfer case/transmission marriage seal

This went bad says a mechanic who changed my oil yesterday. I'm wondering if anybody has had the same problem and if it's a costly fix or a piece of cake?

I was losing transmission fluid and didn't know where it was going. My transfer case seal went, and eventually the transfer case filled with trans fluid, it spit the fluid up into the overflow tube (which is next to the tranny dipstick) and all over my engine compartment. I thought I was going to need a rebuild so I was happy paying the $200 to replace the seal. The problem was that the vacuum lines for the electronic 4 wheel drive pulled the fluid in too, causing that not to work. Didn't realize that until winter came and I went to use the 4 wheel drive. So I had to replace the lines and the switch (which is located up under the battery) for the four wheel drive, so if you replace the seal as soon as it happens maybe you can save yourself the other repairs. At least that is what cottman told me. good luck.