I've been happy with Royal Purple. If you don't have a 4WP or other dealer nearby you can always order online:
http://www.summitracing.com/search/Brand/Royal-Purple

 

when should you change your differential fluid?

 

"Rear axles and power take off (PTO) units containing synthetic lubricant and light duty trucks equipped with
Ford-design axles are lubricated for life. These lubricants are not to be checked or changed unless a leak is suspected,
service is required or the axle assembly has been submerged in water."

http://www.motorcraftservice.com/pub.../08frdmg3e.pdf

 

just thought Id chime in on this one I had the same noise it was the pinion bearing the first time and last week changed the pinion Bering and the carrier bearings.Also keep in mind the truck has 56000 mi

 

I used Royal Purple in my Tundra, but have net yet changed the fluid in my '07 FX4. I have 38,000 on the F-150 and was considering chanig it pretty soon. I don;t know the recommneded changer intervals on the f-150 but Toyota recommended to change the fluid every 30,000.

The link that YJay90 posted didn't work for me. But I have a hard time believing that Ford recommends to never change the rear diff fluid.

 

what weight of oil is recommended for the front and rear differential