Please help me diagnose a vibration in my Dart.

Correct, the caps should not be loose enough to not need some type of press.
Also, the front joint can be out of phase. Chances are you installed it backwards, and if you checked the runout of the shaft you would see a difference depending on which way it was installed.
The only real fix is a new yoke and rebalance. If its a 904 with a 7260 yoke they are still avail new. Heck, I may even have on in own inventory..