transmission fluid leak

First off sorry if this question posts 2x times, I tried to post it earlier but I didnt see it listed so am trying again.
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My transmission fluid is leaking into my engine/engine oil. There is not exterior leak. I was told its probably coming from the seal between the transmission and engine.

My questions are: Does that sound likely? What is the name of the seal between the transmission and engine? How hard is it to replace ~ can i do it with basic tools and a space to work in? If not any idea of the cost to have it done?

Thank you for the help!