Yours is a part time. The full time had a NP203 t-case and was always in 4wd, you could not unlock the front hubs as they spun all the time. Hiloc-Hi-N-Lo-Loloc. It had a chain in the T=case that wore out, small front wheel bearings that were ugly to change. Others may have different opinions but that is my thought on them. Your 73 is a tough truck too, it just has drums on the front rather than the discs on the 80 and up part timers.
I really like the 72-80 body style so the 80 is a bit of a holy grail to me with it's great styling and Part time 4WD system. Nothing wrong with the 81 and up trucks either of course but I just like the early body styling more.

Sheldon