Of all the parts I have both on my trucks and in the garage (or various other storage areas around home), I don't have a single NP205. I have at least 4 NP203's and a 208.
I know that an NP203 is a full time case (that can be converted to part time). However, what happens if you remove the full time locks on the front axle and install lock-outs like Warn? I have done this on my 79/83 plow truck with a 203 and all seems to work well. My 84 3/4 ton that I am working on already had Warn lock-outs on it when I bought it from the previous owner and it has an NP203 case. Doesn't this essentially give you part time 4x4? What (if any) damage can result from using lock-out hubs with a full time transfer case and how hard / expensive is it to convert an NP203 to part time?
You just have to drive it in the lock (HL and LL) positions all the time. If you try to drive in the full time positions, no go. Also, the front driveshaft will turn all the time, but it does in full-time mode anyway.
The 203 transfer case acts like a differential in a rear end. HiLoc and LoLoc act like a locker, of sort. In Hi or Lo if you unlock the front hubs, that end of the diff will turn, causing your rig to stay where it is.