Somewhere there's a buyers checklist, but the major things are: check turbo for dusting (pull the intake off if you can and inspect the compressor wheel), check for fuel in coolant (cracked injector cups), look for oil leaks (around the HPOP at the front of the motor and under the turbo), ball joints, test the 4x4 especially if its ESOF (electronic shift on the fly- no 4x4 and climate control defaults to defrost = vacuum leak), check for rust on the oil pan (if let go they can rust through), and that's about all I can think of right now. If I were shopping I'd lean toward the 2000 unless you know the 99 is a late model... the early birds have a lot of differences that can be a giant PIA.