Suzuki: 90 model samari, diff gears, jp eater

Question
Can you put gears in 90 model samari if so what's best way

Answer
There are option, depending on what you want/need, and what you are going to do with the suspension/tire size.

Here are the options:

t-case gearing changes
  1) stock about 2.23 (I'm not certain of the last digit there)
  2) various kits around 4.16, which also lower the high range somewhat.
  These are usually suitable for use with tires like 31x10.50x15,
  bigger tires climb bigger rocks, increase clearance, and help with traction.
  3) T/case ratios around 6 or 6.5 which usually lower the high range for 33" tires.
Differential geraring changes
  Change the front and rear end gearing - It costs more, but can help if it's like mine,
  I needed the 6.0 t/case gears AND 4.73 rear end gears(4.11 stock) to deal with 34x9.5 tires.
  They probably use up about 20 HP
  While I was in there, I installed lock-rite lockers, which are perfectly useable
  on the street, and last summer I found that I forgot to engage the front end, and still
  climbed a 27 degree shale covered road without knowing I was in 2 wheel.
  They are only about $125 an end, and are WONDERFUL, and unless you're brutal, will last.

I have 6.0 t-case, 4.73 diff gears, lock rites, and short of professional crawlers,
It will do most everything, and occasionally can do stuff (just barely) that a Rubicon can't.

Go to izook.com, and check out the guys on the links
I've used and can recommend rocky-road.com - their jp-eater spring over kit is a joy, and I've had it for five years and love it.  I like trailtough as well, and definitely suggest you take their advice, especially as regards gearing/tire size and so on.  hawksuzuki is a wonderful source for stock parts.
I have the evil twin kit which allows me to go in an out of 4wd without changing high or low range selection.  If you're changing the t-case gears, this is a VERY worthwhile addition to do at the same time.

If you're going to go with bigger tires, it will be to use them at 15psi, or lower for improved traction - beadlocks work, but aren't street legal anywhere in the US.  There's an Australian company that has a very nice street legal alternative to beadlock than I will be installing this spring.

Get back to me and tell me what you want to do, where you plan to use it, and why, and I'll try to get a better series of suggestions.

I 4-wheel mostly in the high Colorado Rockies with the occasional trip to Moab.  I do nasty, steep narrow rocky roads mostly, Sand in Moab, but almost never any mud, unless it's mixed with melting spring snow and ice, which requires frequent winching as well.

Look on youtube, there's a video my kid posted under david scott-Samurai-crystal mountain.  It shows what the rocky road suspension looks and works like....(with a 2" body lift that it had when I bought it)