I’m guessing you have a 3.0 TD6 and not a 4.4 V8? If so, it could be the direct clutch hub that has sheared its drive shaft. Direct clutch is applied in 4th & 5th gears on the 5L40-E.
Drop the oil pan on the box to be sure but our bremble is usually spot on.
Any sign of metal in the oil pan and the box is done.. The oil cooler circuit will be poisoned as well.