Your mechanic friend might be confused by GM's changes over the years.
For example, in '73 they used the NP205 with all trucks, there may have been factory locking hubs. However, starting in '74 1/2 ton and 3/4 ton trucks got the NP203 unless they had a manual transmission, and there would be no reason to have locking hubs with full time 4WD. Then they brought back the NP205 in '80 on all trucks, before switching to the NP208 in '81. I thought locking hubs were a factory option with the NP208.
Bruce