2005 F150 xlt sc 4x4 5.4, 107k miles, 12 bolt rear differential

I just bought this at a very good price knowing it has some issues. The rear differential has made noise for 90k miles. I should add, I bought this from my brother so we know the history. It sounds like a hum almost like a grinding noise. It had a left rear parking brake rubbing so I thought that was part of it.

As soon as I bought it I pulled my Ranger 150 miles back home. I noticed the differential was real hot to touch. When I got home I removed the dif cover and everything looked good. All the teeth were smooth no rough surfaces. The oil was dark but it's been there for 107k. very few tiny flecks of metal in the bottom. I then repaired the parking brake issue, still have the noise. Both axles are tight at the outer bearing, no play. While one wheel was on a stand the other on the ground with the trans in neutral, the wheel off the ground was harder than hell to rotate. the Ranger, one wheel would rotate with ease.

Is this a posi rear end?
(finally the reason for the title) this axle seems large for the truck is this normal or is it a "towing package"?
Any ideas about the noise I'm hearing? It's been doing it for so long it must not be serious but it's annoying.

 

you need to look at the door sticker.....note the axle code
Ford Door Sticker Decoder .: Articles

If you have a limited slip....you might need the modifier additive

 

Thanks! E&B

 

the hum is probably pinion bearings. and sounds like it is limited slip