I doubt there is a simple answer to your question.... what year is your W150? Have you tried taking the vin number to a dealer and asking them to check what it left the factory with? Hopefully it still has the original axles and you'll have your answer.
3.55 gears were the most commonly used gears in pretty much every truck with an automatic transmission. i think there is the odd truck with 3.23 gears and some with 3.92 and 4.10 gears.