There was, long ago, a kit to convert the small lug pattern full time axles to lockouts.

You'll have a better chance of winning the lottery.

IF you already have the larger lug pattern, then yes you can add lockouts, use parts from 85 to 93 plus the hubs, or 80 to 84.

Or plans on a rear axle swap and matching wheels.

Fwiw, keep the existing 203 case and full-time 4wd. You don't save enough to make a conversion worthwhile.