In response to the OP:
How introverted would you say you are?
The reason I ask is that I think when IxxPs are extremely introverted, they may appear to be like Js because they're over-using their dominant function -- which would be a judging function. An INTP who is extremely introverted will probably look like an ISTJ because the functions he is using most is Ti and Si because they're both in the inner world.
The reason why INTPs have the reputation for being "P-ish" is because of the use of auxiliary Ne. But an extremely introverted INTP would probably not use Ne much.
Edit: By the way, from what you've written so far, it seems your dominant function is Thinking.