Some JTs have a shock that go from the center of the rear diff to the body. Maybe that's part of it. I don't have one, I think maybe it was only early manual ones? I never researched far enough to decisively figure out the rhyme or reason some had it and some don't.
Like already mentioned, it is a shield for a axle shock that goes on the rear differential. Similar setup is also on some RAM HD truck models.
It is only on "some" manual transmission models to control axle wrap and wheel hop. There seems to be no rhyme or reason why some manual transmission Gladiators get it and other manual transmission Gladiators do not. Seems to come and go in production and has gone in and out of being discontinued on the parts websites.