You can just install a set of lockouts and not use a part time kit in the NP203. The problem is that you have to put it in Lock (High or Low) to get it to move with the lockouts unlocked because the differential in the NP203 will send all of it's power to the front axle due to it having the least resistance.
Even then, when you are in 2 wheel drive, you are still turning the front drive shaft, the ring and pinion, and the axle shafts. It just doesn't drive the front wheels.
I've done it before, and it is better than leaving them full time. I hate the way they drive when in full time.
As far as Yukon lockouts. I have a set on my 1990 K5. They are hard to turn, and annoying to assemble. If you don't plan on hard core wheeling, just stick with a Warn or even a generic lockout. I have never had any trouble with Warn Lockouts (other than when trying to remove them and they had been on so long that they were too corroded to remove).
Oh, and I don't understand the love for Spicers. They suck. Hard to assemble, hard to turn, some have crappy plastic knobs that like to break.
Martin