The aftermarket considers this an 8 inch diff. Ford is being specific on the ring gear diameter. The diameter is determined by the number of teeth that are on the gear. If you were to get a different gear ratio it may measure 8 in or it may fall in between 7 3/4 in or 8 in. To get parts just refer to it as an 8 inch.
The backlash can be adjusted. The procedure is in the service manual. Rebuild kits are available that contain all of the bearings and seals. The ring and pinion gears are available in different ratios separately. A shop that specializes in drivelines should have what you need. These shops usually make driveshafts also. The sales person should have the answers to any questions that you may have. I would provide any info that is on the differential, though. If you are lucky, there is a small ID tag still attached to one of the studs on the diff. If not, write down any numbers that are cast onto the diff housing and the pinion bearing retainer. These should give enough info for the correct parts or just pull the darned thing out and take it to the shop. They'll then know for sure what you need.
If you aren't making over 300 HP, I would definitely stay with the 8 in. If you aren't able to understand how to rebuild it, let the shop do it. Don't hand it to just anyone, to save money, make sure that they really know how or else there will be guaranteed problems. | 
