WHAT SIZE REAR END GEARS CAME IN THE 1989 SUBURBAN 1/2 TON WITH 4 WHEEL DRIVE

to know for sure the info will be stamped on the axle to the left of the diff cover. Get that info and call the dealer, they will tell you what gear you have. BUT the dealer might be able to give you that info by doing a VIN check, before you crawl under the truck, you might want to call the dealer first.

Agreed, Ford used to use metal tags, which often were rusted off or missing, for axle ID. It may be unreadble on your Chevy, so I would contact the dealership, which will be able to tell you without getting your hands dirty.