Agree with whisperer ... take the olds ones with you. I don't think I have EVER been handed the right u-joints the first time around. You'll save yourself a trip back to the auto parts store if you compare them to your old ones right there at the parts counter.
IMHO, I would look in the phone book for a driveshaft or driveline shop and take your u-joints in hand there. They will probably carry "Spicer" or another name brand...plus there are a couple of different styles that are interchangable depending on your application. Basically, you will pay about the same money, but you'll get an expert helping you and walk out with a name brand product.