What is your desired highway cruising speed and at what RPM? Do you want to cruise at 65MPH and 2800-3000 RPM? Or, would 65MPH at 2200-2500 be better for you?
Since you have H55 and a turbo diesel, a great combination is 3.70 gears (FJ60 diffs) and 33's for good around town power and good highway power without pegging the RPM's. That's what I run on my HJ47 Troopy with 1HZ-T and H55F and am very happy. Lot's of others on the forum run the same combo in a turbo diesel.
So, going to 4.56 or 4.88 and 31's will only slow you down on the highway and probably give you way to much low end torque. In general, you would go to 4.56 or 4.88 if you wanted to run larger tires like 35's. HTH!
P.S. Detroit in the front is not a good idea because of the way it behaves on the street when making tight turns and can cause serious front end wear unnecessarily. The rear behaves weird too and chirps when turning sharply - again, unnecessary friction and wear. Really, ARB lockers are the best for your application since you will rarely need to be locked in the rear much less fully locked front and rear. Plus, you'll have onboard air.