There's nothing in the VIN of which I'm aware. There should be a label on the door jamb with GVWR front, rear, and total. It's sometimes a metal tag (it was on my '78). If it's ex-military, odds are pretty good that it's a 3,800/4,000lb axle but it's no guarantee. If you can't find the GVWR tag, there's no way to know without component comparison. Pretty simple, though: remove an old shock. If it's 13.375" long extended, it's the lighter axle. If it's 15.5" long, it's the heavier one. My '78 D200 was a Slant Six ex-USAF truck and had the heavier front axle. 15" wheels would not clear the brake calipers.