I think this mispronunciation disease crept into Urdu speakers from Persian (given the derived heritage and common side-by-side use in 17th-20th century)
In Khazain ul irfan of sadrul afadil. Surah fatiha. مسئلہ : ضاد اور ظاء میں مبائنت ذاتی ہے ، بعض صفات کا اشتراک انہیں متحد نہیں کر سکتا لہٰذا غیر المغظوب بظاء پڑھنا اگر بقصد ہو تو تحریفِ قرآن و کُفر ہے ورنہ ناجائز [slightly off topic but... A deobandi wrote a book aiming to refute khazain ul irfan and one of the points he picked out was the ruling above. This work by the deobandi was brilliantly refuted by allama ghulam Rasool saidi in Tawzih ul bayan)
Mulla Ali Qari (ra) also states that carelessly/purposefully mispronouncing words when reciting Quran is Kufr. I do not have the reference at hand but was told it on numerous occasions by someone who is a properly qualified Qari and Alim. Wahhabi Imam's in some parts of Pakistan and the wider Sub Continent still pronounce ض as zaad; you will see this if you go to Potha Sher and such places.
he says there's a difference between mispronunciation that is 1. purposeful and 2. unintentional and the rulings will differ. he also reiterates the qayidah kulliyah about whether a mistake alters the meaning or not [bahar e shariat has extensive rulings on this with examples]. he talks about the different rulings of altering letters in ghayri'l maghDubi and wala'D Daallin.
he fully refutes the wahabiya, who in his time at least, promoted purposeful mispronunciation of Đād.
i read both these monographs of Alahazrat: Niýmu’z Zād li Rawmi’d Đād Iljām as-Şādd án Sunani’d Đād and saw that he is also a master of tajwid. he thoroughly explains that every letter is different and purposefully mispronouncing is tantamount to kufr. he also explains the true makhraj of Đād and it's more detailed than any other I've come across. both rasayil are on the excellent www.razanw.org
