All four tires should be the same brand/model/size and be within a 5-10% difference across the set for treadwear.
Check to see if all the tires are the same brand/model and then work up to wheel bearings. Tthe front and rear diffs are standard open units and the center diff is a viscous unit.
3/32 is the published spec for treadwear differences.
Also your info is incorrect.. the center diff being a viscous unit is only true on some manual transmissions.
automatics used a MPT clutch mostly.
I believe the newer 3.6R's use a viscous unit, and they are fairly common now that they put the motor in two of their more popular vehicles. The viscous unit may have gone bye bye with the change to CVT!
As for the tire spec, the data I have is roll out, but my tires are probably bigger than most. My data is for 255/55/18, 0.125 side to side is max acceptable difference total diameter, on the same axle, same brand tires all four corners. Front to rear 0.250 max, Larger diameter need to go in the front! "Subaru Tribeca"
Also, does mention measuring tread depth but is vague and says straight up 1/8 inch under wear in any tread depth from the other tires is out of spec. 3/32's Put them in front if a pair match.
I measure my tires closely every winter when I put my snows on, and its time to do that this week I believe!