in your glove box, there should be a sticker with a whole bunch of 3 letter/digit codes, known as RPO codes. They list the options/features in your truck.
It'll have GT4 or GT5 (most likely GT4) indicating you have a 3.73 ratio.
If you want to know info about the rear axle itself (like if it's a 10.5" or 11" or whatever), the RPO codes don't have that level of detail, someone else should be able to tell you what it most likely is, or you can look at the bolt pattern of the diff cover and measure it to figure out what size it is.
2004 GMC Sierra 3500 C&C, added 4WD, 6.0L gas, Extended Cab, SLE, Transgo 4L80e-HD2 kit installed
Whee! Dually Club Member #1109