With boxes age and mileage don't always factor in. I have been sent boxes with under 40k miles that had worn out gears from being over adjusted, boxes with over 100k miles, never touched and the gears were still good. I've had rebuilt boxes sent in, "new" boxes sent in, I guess to make a general statement each box is different. They are common in the fact they are usually sloppy from use or misuse, but all have been brought back to better then new condition, many needed gears.
There no real magic here just some good machine work and assembly practice is what I use. It take longer to do but the effort is worth it.
The best thing you can do if you're unsure of the condition is to remove the box and put it in a vise, find someone with a 0-30 in/lb dial wrench and see what you have. If it's like most I see you'll fine about 3-4 in/lbs lock to lock. The preload is probably off, but if you follow the thread and GM manual you will be able to see if you can dial it in. Then check for play sideways and up/down. Removing all the play while correctly setting up the preload and lash will make a big difference in the box, night and day. may change your mind on some of the other more expensive options as well.
good luck, it's just a machine you can fix it!