* Ford 9-inch: This is a very common and durable heavy-duty axle. Many F-250s of that era used this. It came in various configurations (different gear ratios).
* Dana 60: A stronger, more robust axle often found in heavier-duty trucks and those with options like four-wheel drive or higher towing capacities.
To know for sure which rear end is in *a specific* 1991 Ford F-250, you'd need to check the vehicle's documentation (owner's manual, build sheet if available) or look at the axle itself (usually stamped with identifying numbers).
