The NV125 was used from 1999 to 2003. But here is the thing, the LWX-500 was also used in 2002 and 2003 models and in 2003 a new electric shift t case was introduced to replace the the other two. In short if you have a 2003 you could have one three different transfer cases.
The first thing I would look for is a shift motor like the picture below. If you have a shift motor than you have an ATC-500.
Once you have determined you do not have a shift motor than you need to figure out if you have the NV125 or the LWX-500. I have seen the LWX-500 in 2002 and in 2003.
For beginners the first place to look is at the rear of the transfer case. The tag for the NV-125 or the engravings for the LWX500 will be located in this section.
On the inside we will look at one of the more obvious differences, like the planetary gear.
As you can see from the pictures above the planetary gear for LWX-500 has a shaft integrated into it and the other one is just the planet. Both have three gears each.
Another internal difference between the two is in the rear case half where the back of the front output shaft sits. The The New Venture 125 has a smaller pressed in bearing where as the LWX-500 has a bigger bearing.
On a different note, the new fill and drain plugs for this unit are updated to bring a small magnet built into them.