Yep. AFAIK, at least with the 8.8", the only 28 spline units were found on 1990+ Rangers (4.0 only), explorers, Aerostars (also 4.0 only), Bronco II's (maybe, and Fox bodies. Everything else (F series, E series, and Broncos at least) is 31 spline out of the box.
1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).