When my father had this problem on the rear bumper on an old Ford van, he borrowed his brother’s sandblaster, sandblasted it, primed it, and painted it. If you don’t have access to a sandblaster, you might get some sandpaper from the hardware store or an auto parts store, sand it down, prime it, and paint it. Walmart has a product called rust converter, which, after you sand the rust off, makes a nice primer coat. Then you could use a spray painting machine and some Rustoleum or just a can or two of spray paint. If you remove the bumper, or mask the surrounding areas well enough, it might look halfway decent.
I had a '98 Sierra a few years back. I think a brand new chrome rear bumper with hitch was less than $200. I just checked Autozone and they are still priced around $200.
You may not be able to really fix it if parts are rusted away like the bumpers on the east coast and snow belt areas or Cleveland. A body shop will charge around $600 to order and install a chrome one which costs less than a painted one. Of course changing it yourself is cheaper.
Depending on how bad it is, a Brillo pad will clean all the rust pits up and make it look better. I used JC -itney miricle silver paint on my old VW that the chrome was all peeled off on. Looked better but still looked painted. My BIL used Denver Bumper for some rechroming on an old car and they did a great job and sounded very reasonable. Myself, I’d just be hitting it with Brillo and chrome polish unles the chrome is peeling.
grizz im not sure where you live but i find it very hard to believe you cannot find a used bumper for this vehicle they share the same bumper with the trucks of that same body style and do you know how many of those are now and have been on the road? LMC trucks list them from 199 to 319 dollars. i also double checked the part numbers so i wasnt giving you bad info they are the same bumpers that are on the trucks.
Good advice from all. Let me add that it may also be rusting at the mountings. If still solid, coat these areas with red grease using a foam paint brush. Replace the hardware that isn’t solid. That’s more important then the looks.