5.12s are the largest ratio gears in the newer Trackers, however some earlier model Sidekicks came with some steeper gear sets, it is just rare to find a set on the market for sale. The '91 4-doors with manual transmissions had 5.38 gears and the '89 models with the 1.3 carburated engines came with 5.62s. Again, rarely available in the classified sections and when they do show up, command prices of around $600 for the 5.38s and $900 to $1,000 for the 5.62s.
The only other option available that I am aware of is ordering a new set of 5.71 gears from 4X4art out of Turkey for around $1,000 to your door. The quality on these are not known yet. A forum member purchased a set of them last December. When he opened the package and wrap they came in, the "new" gears appeared to have already been installed and run, at least a little, on another diff. Last he posted he was having problems pressing the pinion bearings onto the pinion shafts. That's a shame too, because the gear market for these trucks is wide open right now for SOMEONE to dominate. A person could probably sell a few hundred gear sets a year IF they provided a quality set of gears.