Another common thing, no matter how experienced the installer, is an incorrectly routed belt. All it takes is one or two wrong paths to give some issue. It might look right, but could still be off.
The ZJ's aren't that intricate though, so while it's possible to do, it's probably not your issue.
But it's worth triple checking I think.
I always buy the Conti or Goodyear Gatorbacks nowadays. Don't even ask the price anymore either. "Just give me the Gatorback" is what I say. I like the unequally offset grooves & notches that reduce harmonics, so that even when there is some slippage, the noise factor is way down.
This might be a two-edged sword of course, reducing the number of symptoms you might get before a failure, but I hate noise. And cleaning glazed pullies, while an unfortunate necessity of older rigs sometimes, is also not on my favorite things to do list.
What about some contamination? Maybe your previous work left some coolant residue on the pullies and it's just now coming back to haunt you with the new belt?
Or even something from just this time, like oil or more coolant or refrigerant maybe?
Might be worth a simple cleaning to see if it goes away.
Is yours a V8 or I6 101? Since the sixes are manually adjusted for tension too (are all I-6 ZJ's this way?), could you just need to tighten the tensioner some more? I don't know that a too-tight belt would squeal, but a too-loose one certainly will. Unless you're concerned that it's too tight already, and too tight does put more stress on the accessory bearings, maybe another turn on the adjuster is called for as an experiment.
I actually don't know the proper method of checking tension. I've probably just gotten lucky when I've done them before. No squealing so far though. Knock on wood...
I prefer automatic tensioners for simplicity, but you can't argue with a set-it-and-forget-it type much either.
Good luck. Noise sucks.