I don't think 4.30s were ever an option in 99+.
4.62s and 5.12s were only available with manual trannys, from what I have read (4.62s with the 2.0 and 5.12s with the 1.6).
4.88s were the only option with the auto, from what I've read.
Is the vehicle where you can go look at it? If so, just jack one rear tire off the ground, spin it exactly 2 turns and count the number of times the driveshaft turns. The number of driveshaft revolutions in two turns of the rear tire is the gear ratio (4.62 turns or 4.88 turns or 5.12 turns).
I got my info from the following link, as well as a couple of other sources....
http://car-part.com/Look up 1999/chevy tracker/axle assy rear (w. housing). It will then ask you to select the ratio, based on the tranny and engine.