i have a really bad vibration at about 30-60mph and i cant get it figured out. any ideas? i have replaced the parking brake and driveshaft u joints

 

Had you balanced your wheels? do you have bent wheels? are the tires cupped? try rotating the tires. If there is a change then you can trace the problem to wheels?

 

I'm experiencing the same. When the shaking gets to where the truck seems to have severe wheel hop, around 40 - 50 mph, I need to put on the brakes and slow down for it to go away. It could be anything from the transmission back. I'm starting with the rearend, focusing on the pinion bearings. When I bought the truck, the pinion seal was leaking. Ford tech replaced it and said the rearend was a quart low, who knows for how long. He had no idea how long the bearings would last. Total rearend rebuild is this summers project.
If you find what's causing it, please let us know.

 

it was the wheels the back 2 tires were about 4 ounces off a piece lol

 

Took my truck in for a wheel balance check and three ounces off on each rear tire. The simpliest solutions are often overlooked!! Dohhh!!

 

I know this post is a month old or more but just wondering if you had it fixed. It could be that your driveshaft is not balanced as well. I have a little bit of the same issue. After replacing the U-joints, I immediately noticed a little vibration between 50 - 60 mph only. It goes away at any higher or lower speeds.

I figured its probably the driveshaft and will probably get it balanced (and I did join the driveshaft joint knuckles back to the same way they were originally)

 

Vibration at those speeds is very often just tire balance or out of round.