If you mean the axle shafts, the snap rings are on the inside of the yokes. they're semicircular, rather than the doubled-over spring clips you're used to on driveshafts. If your u-joints are rusty it may even be a little difficult to see them. It's a PITA sometimes to get these things out, even when they're not rusty. You need to prove until you find one of the ends, and then try to sort of rotate/flick/pry/wish/cuss/levitate it out of its groove. At least they're easy to get back in.
Look at the new U-joint. If you have the right one, you'll see that the inner end of the cups is grooved, and there should be a bunch of the rings there to study.