After 205K miles, damage may have been already done. However, in the two years I've owned the truck, I haven't heard any whining or clunks from the rear end. The old lube contained no metal particles that I could feel, and ring/pinion backlash is around .012". I changed the axle seals, but the wheel bearings looked and felt OK. Ring gear and spider gear teeth looked fine.
After starting this thread and reading your comments, I've done a little research. Turns out my truck's differential (8.5" 10-bolt) was first used by GM in 1970, However, GL-5 wasn't introduced until the mid-1970s. I doubt GM redesigned the diff for use with GL-5, but instead simply specified the latest and greatest lube. It's kind of like specifying SG motor oil when it became available instead of SF.
All that said, I do agree that GL-5 would be better.
Thanks for your comments.