78 was not the first year for the 31 spline axle. They were optional in earlier years. There were more years than just 76 and 77 too. I think it was an option back to 1971. The vehicle tag will say something around #3200 for the 31 spline axle and #2850 for the 28 spline axle, but I know for a fact that Ford put 31 spline axles in the little Bronco.
78 was not the first year for the 31 spline axle in a ford truck - but it was in a bronco. it was not an option in any bronco until 1978, when it became standard. there were different rated axles, with different sized bearings, brakes, and housings, but sadly, the 28 spline axle was your only choice in a 1966-1977 bronco. check out this url for more info: