Looking back at your original description of the problem, I've never had a vibration that was horrendous or changed with applied power except the front of the drive shaft. Slack in the rear of the drive shaft will give you clunks but not a horrendous vibration. Here's my suggestion- With the car safely secured in the air, trans in neutral, hand brake off, crawl under the car, grab the drive shaft near the front yoke and push, pull and twist in all directions and try to see any movement then rotate it and do it again. There should be NO MOVEMENT except rotational. It only takes a few thousanths of an inch to give you a terrible vibration that will change with power application. Other possibilities-the u-joints that were installed will not balance. The front yoke was damaged during u-joint installation or not installed in the same position in relation to the drive shaft. The old c-clip was used on the new u-joint. The drive shaft is bent.Another club member, a while back, said that he had found a problem with the out-put shaft in the rear of his automatic. By your description this doesn't sound like it is relatable to wheel speed. I've had some severe wheel vibrations but they pale in comparison to drive shaft vibrations because the drive shaft turns about 4 times faster than the wheels and axles. Mark in Portland