Here is some good info on the 203 case and part time conversion -->
http://coloradok5.com/milemarkerpt.shtml
Partway down on that page they describe how the front output is engaged:
Quote:
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The way the NP203 performs "LOC" or Locking to drive both axles with equal power was with a sliding collar on the top chain gear. The collar would slide over the crown gear and engage teeth on the differential case. This locked the top chain gear to the differential and would not allow the gears to spin giving a 50/50 power split. Once the transfer case is converted then the sliding collar becomes the only drive to the front drive shaft.
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And the pic from that site that goes with the quote:
So it's possible that the teeth on that lockout clutch mechanism are worn/damaged to the point where they are no longer holding under power.