The one in the picture looks older than an '81. '81 was the 1st year of the slanted nose and square headlights (except ofr the factory option recatangular headlights in some trucks in '80 models). But anyway, looks like an '80 or older truck by the pic. '80 still had the vertical side marker lenses, '81 had horizontal marker lenses.
Shifter handle in the cab should identify the t-case w/out having to crawl under. Round handle indicates a 205, handle with flat sides indicated a 203 and a round handle that angles foreward with a console around it attached to the floor that lists the t-case positions is a 208.
If it's an automatic, I'm sure its a TH350. Most likely the whole truck will be as follows:
350 motor, TH350, NP203 (or 205), 12 bolt rear, and Dana 44 or 10 bolt front.