When installing u-joints, after you install the first two cap's, check to see that the joint rotates freely. New Joint's should NOT be stiff to turn. If you feel some binding, hold the new u-joint ( or yoke ) in your hand (shaft NOT in a vise) and, by using a ballpeen hammer, hit the yoke between the cap and the weld. Be VERY careful here, as you don't want to distort the bore for the cap, and you don't want to hit the weld on the shaft. Check the others as you assemble them. A few hits in the right area will 'set' the cap's and you'll have a longer lasting and vibration free joint.
You'll know when it's right when you can move the joint with 'baby finger' pressure. Don't forget to grease it when your done, as the grease(?) it's shipped with, is just that, Shipping lube.

I got my new u joints from Advance auto called Brute Force. They come pregreased with synthetic lithium grease. Looks like some good peices. Always remember to install the inside clips.

Yoy're right about the pre-lubed/premium joints Tommy, and I should'a said something about the clips. Years ago I left one out or didn't get it seated and, well... you can bet I had a wake-up call going down the freeway.

If you are making any kind of power above stock I would reccomend Spicer Ujoints they are about the strongest I know of that are reasonable on pricing and always check to install the grease fitting at the right location (I am having almosthiemers and can't remember the right wording for this there is a weak and strong position to position the grease fitting in relation to the the yokes since that is a weak point in the cross of the Ujoint.