I doubt the lube was even low. They most likely filled it until the fluid was coming out of the fill plug when there is a "fill to" line on the back cover of the AAM axle which puts the "full level" about 1/2" below the bottom of the fill hole. You'd have to run a diff pretty low to run into any issues with low fluid damage. Lot of "splash" oiling going on in there.

 

Yeah, and even after all the gear oil leaks out, it'll still drive loaded on the interstate for hundreds of miles before the diff turns into a "locker"

Then you can add some more gear oil at a truck stop and stay puckered 'til the noise subsides. If your nerves survive the excitement of the clutch packs (I know - it wasn't an AAM!) unwelding themselves on the interstate (one side at a time ), you'll be in a good frame of mind to handle it a couple of weeks later when the heat-stressed spider gears explode THROUGH the diff cover.

Brand new posi carrier, rebuild kit(bearings, seals, etc.) & labor set me back $1200...

 

I'm guessing you have completely ruled out the rear u-joint haven't you. Remember never to overlook the obvious or the simple solution. I had one that went out in a 02 F350 dually once and thought the darn torque convertor was shot, and I have a ton of mechanical experience. The only way I figured it out was by changing the u-joints just to make sure it wasn't something silly like that.