The rear u-joints starting to go, you would tend to feel the vibrations comming through the seat, not the steering wheel.

Since you have already replaced the upper/lowerr ball joints, pitman/idler arms, and tie rod ends there isn't anything left in the steering linkage, except the steering stabilizer. A worn stabil. wont cause a vibration, only a drift while driving down the highway. You could replace it, should run you under $30 for a new one.

With the upper bushings being worn you are going to have to change them(lower ones too while you are at it), in order to get good tire wear/milegae warranty out of them. I still don't think this is the problem, but you never know.

I would say it is in the tire/rims, try moving the front to the back and see if the problem goes away. If it does you will know it is either a rim or tire.

How many or large weights were used to balance the tire? A lot of times tire guys will just keep adding weight where the machine tells them top get the tire balanced at 50mph, when all they have to do is move the tire on the rim to offset the out of balance. You could also have a bent rim(s), which would cause a vibration, most tire shops wpont catch this unless it is really bent.