I have been searching and can't find an answer. I got a replacement rear wiper arm for a 99GC. It is Dorman 42892. The hook is much different than the factory; it makes the U then stops, no little attachment tab. Here is the closest thread I could find and the last post shows the different "U" hook.
I recently replaced my front wipers with Trico Neoform blades. They are installed the normal way but they have a tab that clips over the hook and that's what holds the blade in the hook, not the traditional little tab that goes into the hole that you are missing. I don't know if they make the smaller rear one.
I have found that Bosch Micro-Edge are good wipers. They last longer and keep a better wiping edge that Anco and other wipers I have tried. They clip on, as most wipers these days, with clips on the end, that clip over the end of the "U" shaped part.
It appears that at some point in time Jeep changed the rear wiper arm design. The Dorman arm looks exactly like the arm on my wife's '03 WJ that I replaced several years ago with a Mopar arm.
The Dorman arm fits just fine, however on older WJ's you'll need to buy the little plastic cap that covers the wiper arm mounting nut from a later WJ along with a matching wiper, since the hook attachment is different.
The thing that bothers me the most about the Dorman arm is the plastic tab that rests the arm on the tailgate is pretty cheap and will likely break easily. I also don't care for the section of metal that actually attaches to the wiper motor stud as it just feels like cheap pot metal, however that could be due to the high gloss powder coating.
For less than $8 on Amazon, its acceptable, but for around $18 on eBay you can buy a new legit Mopar arm.
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Ok, found something interesting out. Went to the auto store and had them pull up a pic of a 1998 GC rear arm. It looked identical to the one off my jeep and even had the little clip on the end of the U. It is dorman #42589. Looks like it will work, will see tomorrow.
Went in today and looked at the 1998 arm. The are where it bolts on is the same as my stock unit and it had the U with a hole. However, the plastic piece for the return bracket was too short. Ended up staying with the arm I originally purchased, got the wiper blade from the dealership, and the end cap from lowes. Worked out pretty well. I just hate the wiper blade lying on the paint, bugs the heck out of me. I think I may make a ramp for the bracket to push the blade further away from the back.