Yep the ford truck nine inch is a 31 spline 5 on 5 lug pattern and the car nine is a 28 spline 4 1/2 on 4/12 lug pattern.
Hate it when that happens
Depends on how old the 9" is. My 63 F100 is 28 spline 5 on 5-1/2. I think 28 spline went to '66 in trucks? Then went to metric bolt pattern sometime in the 80's. Also I think the cars usually had a centered pinion meaning the axles would be two different lengths. Lincolns, Mercs, and cop cars often have a different lug pattern - 5 on 4 maybe?
Dealing with Fords ain't for the simple minded, that's for sure.