JD, there were all kinds of possiblities back then but I don't think that's one of them. After all these years you may find W100 trucks with a D60 rear/D44 front, like a 3/4 ton, but it's more than just axles.

One I owned was a '78 W200 with a Dana 60HD rear end, which is a D70 housing & axles but a D60 ring & pinion, plus it had heavier springs. Standard issue was a plain Jane D60 rear, so these like I had were called 'Heavy 3/4'.

I don't know of a 60's W100 coming with heavy duty drive train. Another confusion: I did a body swap with that truck, mounting '78 D series body on '77 W series chassis. Anybody can do that, but you're supposed to swap the VIN tags as appropriate, to identify vehicle.