Wikipedia has good pages for mercon and dexron, but not for any Mopar ATF oils. My vote is ATF +4. If you're just topping up, I don't know if it "safe" to add +4 or Dexron to what could be already there. If you're draining the whole thing, I'd go with +4.
Regarding draining the trans, I got this from a 2015 post from Bob's the oil guy:
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Just disconnect the line coming out of the trans cooler, stick it in a 5 gallon bucket, and have someone start the (engine). You may have to put it in neutral to get the fluid circulating. (oil will then be flowing out the open line). As soon as the fluid starts "spitting" have your assistant shut the (engine) off. You can now drop the pan with out making a huge mess and you'll have evacuated about 2 gallons out of the system (give or take a quart or so.) You can loosen the valve body and let it drip over night if you want to get another quart or two out of it. Even leaving it sit over night with a catch pan under it will net a little extra. This is the only way I do a trans flush/fill. Dropping the pan only gets a fraction of the fluid out of the system and then every time you drop the pan after your draining partially new fluid.
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