I don't know of a web site that lists the codes, but you can tell pretty much anything you want to about the axle by looking at a few things. You said that it is a 3/4 ton, which would make me think it has one of two axle assemblies, either a 10.5" ring gear full floating 14 bolt, or a 9.5" semi floating design. If it is a 6 lug 3/4 ton found on some newer rigs, it's the 9.5". You can tell by taking off a hub cap or wheel and seeing if the hub portrudes outside of the break drum. This is a full floating one if it does. The full floater also has a more angular diff cover, with a dip on the right hand side of it, while the semi floater is more rounded. You can find your axle ration by taking off the diff cover, and dividing the number of teeth on the ring by the number on the pinion. Good luck.