INTPs have a judging function, which is internal (Ti). It is their primary function, and it makes them judge things based on their internal perspective of truth (this is true because it makes sense to me). INTJs have an external judging function (Te). It makes them judge things based on their external perspective of truth (this is true because it is the widely accepted theory/fact).
We only use the letter "J" at the end to represent a type which has an external judging function (because you'd probably think of someone who goes around judging other people as more judgmental than someone who judges themselves). All types have the ability to judge things. But an INTP would be more concerned with judging their own actions/ideas, while an INTJ would be more concerned with judging the things/people in their environment.