As Rick said, measure the outside diameter of the carrier side bearing's outer race. In addition, there should be some part numbers on the old bearings. Those numbers can be crossed over to a replacement set/kit also. With the 28 spline 9" you're probably running the small OD bearings anyway. Don't forget to check if you have a crush sleeve or solid spacer. The install kits all come with the crush sleeve. If you have a solid spacer now you can reutilise it with the new setup. You might want to buy a solid spacer shim pack though as different shims may be required when resetting the pinion preload.
You'll be measuring the OD of the upper/lower lefthand picture.
