Hmmm, I don't know, but from where I stand, it still sounds like it could be your driveshaft. Why don't you check the U-joints again.
Then again, have you checked your tire pressure? Also, have you checked to see if (by running your hands around the tires) if there are any bumps or warps on the tires? Start with the easy things first (of course
).
Concerning a harmonic balancer: look at the balancer while the car is idling. It should turn straight with no wobble. After this, if that's fine, shut off the car and inspect the balancer from where it is. Try to look CLOSELY at the rubber seal/ring on it. It should be even ALL the way around the balancer. If the rubber is gone, heavily cracked and rotting, or significantly uneven anywhere on the balancer, it should most likely be replaced. I bought one a long time ago from Ford dealer for a little over $90 (I think) which I think is a great price.