Heres Snap-On's version, and since its Snap-On they want 300 green ones. But heres what they look like, they all differ a little bit but if you get an off brand one it'll still work good.

http://buy1.snapon.com/catalog/item....re&dir=catalog

Pretty much a c-clamp that an impact will go on. Has a hole on the end for the u-join to come out of. Makes REAL quick work of it, but if your not kinda carful and go to fast you can bend the shaft a little, and thats no good. But use some brains and go slow on the impact and they work great.