Miles
We just bought our son a 99 Limited 4x4 with 189,500 miles on it. One-owner spent all of its life here in Phoenix, AZ. Had our mechanic check it out and give it a clean bill of health. I don't think it's ever been off-road even. Yes, we did engage all of the 4x4 modes including the rear diff locker and it all engages. I was even able to get the local dealership to fax me all of its service records - added bonus to know some of its background.
We'll start using full synthetic fluids in it as we begin changes...
We're expecting it to last him 4-7 additional years putting @10K miles per year on it.