odd thought. Could you replicate the grinding noise on the workbench? Basically, assemble say half the drive train (slipper to rear) and give it power and see if it grinds, swap diffs in to test.
Speaking of diffs, have you opened any of them yet and confirmed (pull everything out and then check, not leave assembled) the gears, gears to input shafts connections, etc.
As for the slipper, if you have the exotek locker on hand, try that too. Could be a good way to eliminate one area.
Also, confirm the motor is good to go too in another vehicle.