Jane Austen most assuredly uses high diction in Pride and Prejudice, although Kitty and Mary are not above using middle diction that is sprinkled with colloquialisms. Diction differs from vocabulary in that vocabulary indicates the individual word choices whereas diction refers to the whole collection of word choices. In other words, high diction might contain some of the vocabulary words in low diction, such as cat, street, love, honor, while the recognizable diction differences would remain in tact.
There are several kinds of diction categorizations. These may be high diction, middle diction, low diction, concrete diction and abstract diction, poetic diction. High diction uses an elaborate vocabulary with polysyllabic words and finely constructed grammar and syntax free of errors. Middle diction is the language of educated people and includes correct grammar and syntax but is less particular and elevated. Low/informal diction is the language of casual (sometimes careless)everyday talk and includes idioms, slang, contractions, elisions, with grammatical and syntactical errors.
In Pride and Prejudice, Austen's diction is always superlative, of the highest form, and even Kitty and Lydia make no errors in vocabulary choices or grammar or syntax when they incorporate colloquial expressions into their talk. As to imagery, Austen's imagery is usually of the abstract sort (employing abstract diction). One is "elegant" and feels "esteem" and experiences "disapprobation," which are all abstract concepts.