Wow, I would think if your engine had any punch at all that a 400 would smoke the rear skins off!
To help get you started with IDing the rear axle ratio, you can do the following if you cannot locate the code, or want to check to make sure the innards have not been changed out:
Transmission in neutral, brake off, front wheels blocked, jack rear up by a jack under the differential.
Place your jack stands under it.
First spin one of the tires around, probably the other rear tire will be turning in the same direction. This will indicate you have a safety-trac (posi) rear axle.
Spin a tire around until you have the rear U-Joint in a position where you can mark one cap. If you are turning the drivers side wheel, maybe have the U-Joint positioned to where a cap is straight down at the 6 oclock position. Mark the cap so that you can watch it as you spin the tire. Mark the tire so that you can spin the tire one complete revolution, probably straight down to the 6 oclock position also.

What you want to do is spin the tire one complete revolution and watch and count the U-Joint revolutions. If the tire turns once and the U-Joint spins twice you would have a 2.00 rear axle, 3 complete spins would be a 3.00 rear axle. It will not be exact, but you will be able to tell the difference between a 2.56 and a 3.23, or a 3.23 and a 4.33.
Cal