Quite certain this will be a junkyard item today. I believe these vehicles got Delco brakes, but other than comparing to period GM vehicles with similar brakes, I don't think there is any way to cross to something from GM. Lots of GM cars use the same shoes, but the drums are unique to these vehicles. I would look at the Jeep factory parts books at www.oljeep.com
and see if those backing plates are unique to the GW Dana 44 rear axle (likely) and check the part numbers you got from your online source.
I suggest you contact FSJ Jeep junkyards, such as Montana Overland or Jake's (in NC). Unfortunately Jakes is winding down and does not ship anymore, but maybe something could be worked out if they have the part. There are also Jeep junkyards such as J&W in Antelope CA. Worth a call. I understand there are other Jeep-only junkyards around the nation, including ones in Colorado and Georgia which I don't have any other info about. There's Collins Bros in Texas that has some kind of a yard, but they focus on newer models and I doubt they would have this part.
Try car-part.com and search for a 1989 GW rear axle. I found lots of axles nationwide when I searched, and a yard with an axle is likely to have and may sell you a backing plate.
You should also post an ad in the WTB (want to buy) forum here and at FSJ network, as well as the big all-Jeep forums.
I'd think your local professional welder could fix a cracked backing plate.