There are two basic ways to replace u-joints... You can press them or beat on them...
This shows the hammer method... and if you follow the steps it will work.
[ame=http://www.youtube.com/watch?v=9PpR-DcRV08&feature=related]How to replace U-joints The THOR method - YouTube[/ame]
I prefer the press method, but I think Most folks use a hammer.... KennyV