Count the bolts on the diff cover. Not trying to be a smartass here, but GM drive axles are identified by the number of bolts holding the cover on, so... 10 bolts make it a 10 bolt, so on and so forth. As for gear ratio, do what he said ^.... or, there are a couple of web based calculators that you can google, input your tire height and rpm at 60mph in direct(fourth gear in your case) and it will give you an estimate of your ratio......or, pull your diff cover(fluid could prob use a change anyway LOL) and divide the amount of ring gear teeth by the amount of pinion gear teeth. There are a couple other ways too, but these are the easiest that I can remember.