Honestly, I'd apply a rust converter to stop what rust you have, remove the flaky bits of wax, paint it black again, and coat it with Fluid Film.
AC Delco waxes and rubberized coatings are garbage. Just google it a bit and you'll realize just how worthless this approach is. You'll exacerbate your problem and flush money down the drain. There's no random pattern of GM trucks that have crap undercoating. There's just trucks that have had the junk undercoating fail and there's GM trucks that will soon have the undercoating fail.