[Tim]

99' Rodeo with 59,300k miles and a rear end diff chatter. Can I just add a bit of limited-slip additive or do I need to empty and re-fill?

 

[NwFlTrooperFan]

With that mileage - change the fluid & add the additive. Gear lube breaks down too. It's way cheaper than changing a rear end and the only thing you can do to protect it.