Weird Vibration!

make sure your ujoint on the rearend has the retaining clips on the outside of the caps, mine fell out and i got a wicked vibration.there is a bit of play were the yoke goes into the transmission that is normal. does your driveshaft still have the balancing weight on it? sometimes they come off. you should be able to take the driveshaft in to any machine shop with a lathe to turn it on to see if it is true. good luck and stay with the 4speed ,you will love it.