I need to do have some work done on the rear differential, but the fill plug is frozen in place. The mechanic recommended finding a new Rear differential cover, with a fill plug built in. The cover has a 12 bolt pattern (I think its a 10.25 ring gear size). I've looked high on low and can not find a cover with a fill plug. Any help would be appreciated.

 

You can customize the finish and add a fill or even a drain plug here:
Ford Sterling 10.25/10.50 - Chrome Differential Cover - Ford Super Duty

 

I would bet your axle is a 9.75 inch model... double check the door post sticker or axle tag....... any good welder can add a fill plug to the original cover... would take 45 minutes.

 

G2 Axle & Gear FORD 9.75" Aluminum DIFFERENTIAL COVER , 40-2012AL | eBay