[1969 GMC]

the NP203 is always in 4wd mode, but there are 5 positions on the shifter:
Hi, Hi-Lock, Neutral, Lo, and Lo-Lock.

i dont know of any articles about the 203 offhand, but you could try Googling or also search Pirate4x4 or ColoradoK5.

in a nutshell, the 203 acts as a center differential in addition to being a transfer case. Imagine that the Blazer is just a big axle, and the frontend of the blazer is one wheel, and the rearend is the other wheel. The 203 is the diff. Basically, when in Hi or Lo, the front and rear axles can spin at different rates of speed (much like wheels on an open diff axle can spin at different speeds). You would use either of these positions when driving on pavement, to make up for inconsistencies in the road and cornering...and nothing will bind up.

When driving in Hi-Lock or Low-Lock, now imagine that big axle with a locker in the diff. This makes it so that the front and rear axles spin at the same speed all the time, like how the wheels on the big axle spin at the same speed all the time. This is the 203 equipped truck's equivalent to a part-time 4x4's 4-Hi or 4-Lo. In a part time truck, the axles always spin the same speed, as there is no center diff like in the 203.

Hopefully this doesn't confuse you more...if it does, then try searching, as I dont want to type any more, lol.