80-85 Ford put the leaf TTB on it's F250s and F350s. In 86 Ford started putting the King Pin D60 under the F350s again. I believe in 90 or 91 Ford switched to the Ball Joint D60 and started putting solid D50s under there F250s (could be wrong on the dates there) I think Ford was just using up it's surplus D50 sections from the old TTBs on the solid axles untill they ran out of them. Are you looking for a donor axle or are you looking for a truck to buy?