And you are making a huge assumption that nothing has ever been changed at any time in the past 50 years. These things get a lot of modifications, and sometimes questionable repairs. Some by the home mechanic that doesn't know better, other times someone pays a mechanic that doesn't know any better. And there are times where it is just a quick fix with parts on hand, or different parts that someone has that they think might make it better. So you really don't know what you have unless you check, the numbers on the tag simply tell you what it was built with, not what it has now.
A Bronco axle is specific to a Bronco and nothing else. F150 axles have been swapped, generally called a full width or full size axle swap. Makes the Bronco tires stick out past the body as the axles are about 6" wider then a Bronco axle.
And they were all built, in all years, with a 28 spline axles in the back. Regular, heavy duty, or a medium bearing rear axle they are all 28 spline. Unless someone has done an upgrade sometime in the past. Back to that not knowing what has been done between when it was new and when you have it now. 31 Spline was used by Ford in some of the 9" rear axles, just not the early Bronco ones.
The heavy duty has nothing to do with the differential itself. It is a higher GVW package, an extra leaf in the rear springs, a bigger bearing on the axle to handle the extra GVW on the axle, and drum brakes an inch larger in diameter. That is what you got with a heavy duty axle back in '69. And the placard lists a higher GVW, which in some states moves you up a tax bracket. The ride is worse when not loaded as well.