Not sure I'm following your repair process... you think its the ujoints, but you still suspect the engine mounts? Or are you trying to figure out other possible options?

Also, you'll get more results on your questions if you try to make your questions and subsequent followups as specific as possible. For example, when did you balance the wheels? Was it lately? I've had wheels balanced, and chuck a weight on the way home...

In any event, I've seen instances of a vibe that feels like its in the front, but turns out to be a result of something wrong in the back: rear ujoints could manifest a problem this way...