Ford Super Duty (1999-2016) :: 2004 F250 - Rear Differential Going Bad?

I have a (new to me about 40 days ago) 2004 F250 SD Lariat Crew Cab 4x4 6.0 diesel. 137,xxx miles.

I thought Id gotten a good "deal" on this truck when Id bought it however the last 40 days have been very expensive. The "stealership" I bought the truck from was on a 35mph rd and I couldn't test drive it at highway speeds. So I bought the thing "knowing" there were a few "minor" sniggles wrong... however once about 15 miles from the dealership/stealership and on a highway, the real fun began.

I had a terrible shake, shimmy and groan/whine at speed no matter if the truck was in gear or not.

Since that time ive repaired/replaced:
1) Both front hubs/bearings
2) Both front hub locks
3) Both front brakes and calipers
4) 4 New Hankook tires
5) Ford Dealership Diesel engine service/lube (essentially an expensive oil change)
6) Ford Dealership transmission service
7) Drained (with a siphon) and refilled the rear differential + OEM Ford friction mod.

Ive eliminated problem after problem after problem with this thing.... and yes, the truck does get better with each repair HOWEVER.... Ive still had this groan/whine from the rear end of the truck and a rumble from starting at a dead stop despite my changing out the gear oil in the rear diff.

So I take it today to a local shop (that I trust/sort of) for a 2nd opinion. Their technician takes the truck out for a test run and comes back in 10 min saying he has no clue but the sound was annoying and he'd need to put it on a lift if I could wait.

I saw the truck go up... a few parts get wiggled, some head scratching, saw the truck go down, watched another tech jump in and 2 techs run it out for a test drive.

When it came back, I watched the truck go up again on the lift... this time however they just lifted the rear and with one guy in a pit the other revved the motor. I watched as the wheels spun. A few minutes later both techs came in and said my "rear differential was shot".

They pulled the diff mag plug which was caked in metal deposits. It looked the same when I pulled it off the first time and did the fluid change (yes i cleaned off the plug before putting it back). I had seen differentials (in the past) have deposits like that before on plugs but the units were still good and such deposits were considered casualty of wear and tear.

However im being told the rear end is shot.. When I inquired about just replacing the internals of the diff, it was a simple bolt in/bolt out job.