New owner needs some experienced help.

[Ontheedgeofthecliff]

Any helpful experience or mechanic perspective is being sought out for a new edge owner. 
2016 edge titanium 3.5L 6 speed 142,000 MI. 
bought used from private seller. 
grinding sound present in rear end upon long drive home AFTER test drive. 
sound is loudest at beginning gears and gets quieter once momentum is built. 
does not change during turning, or cornering. 
not present in reverse

when driving it feels like the engine downshifts (going up a hill at 60mph) but the torque is minimal like it’s working against its self when it downshifts, just RPM increase. 
 

history known so far:

owner 1: rear diff and wheel bearings replaced at 91k miles OEM installed by authorized ford dealer 4 yrs ago. 
 

sells vehicle to large dealership and owner 2 buys vehicle at 106k mi. 
 

owner 2 replaces PTU OEM part but installed by a performance shop. 
has vehicle since 2023 after PTU install and lists it this year. (So they drove it for 3 years). 

local shop says rear diff is bad, but does not know the history. I’ve been wheeling trucks and hauling for years and never blew one RDU. I’m doubting the RDU diagnosis if it was replaced in 2022 but honestly don’t know where to start. 

im not a mechanic and i own a ranger so i know nothing about the AWD system. 
this is my sons car after we came back stateside from a country he was not old enough to drive in. I’m just trying to get his car that he saved for 4 years for to be diagnosed. 

please help!