Do you have a manual or automatic transmission?
Here is the tera low kit: http://www.amazon.com/Advance-Adapte...na+20+tera+low
It's for manual trans only, but I think they make one for autos. It's a different input spline.
Yes, it only affects low range.
Here's a good gear ratio calculator that lets you compare 2 setups side by side:
I'm running 35's with 3.54's on my Scout with a manual trans. That's pretty good for the street, but I would like more gearing in low range. Not that it stops me, I just have to slip the clutch more and use more skinny pedal. Any more gearing and I would run out of spare axle shafts quick. (Watch for upcoming thread about my future upgrades)