I don't know much about the gasoline I6 (and what I DO know, I've learnt here!) but on your Reddit post you mentioned fuel trims are off (presumably +) so that kinda points to a big vacuum leak somewhere? Or more than one?
*edit* oh, I forgot to ask... is this only happening when braking (to slow down) or does it also happen if you simply take your foot off the gas pedal? If it only happens when braking, your brake booster might be your vacuum leak.
