why does my van shake from the rear violently?

So without confirming the symptom you are sure that its the U-joint and it can't be a tire out of balance, bent wheel, broken belt in a tire, (bent axle, brake drum balance etc....)

"Real good chance" I seem to recall.

The van has disc brakes. I had the tires balanced and rotated about a month ago. I replaced the u-joint by the differential today and did a very basic wheel bearing check, took the wheel and brakes off and made sure there wasn't a wobble, Hub spun excellent without any play. With the wheels on there wasn't any play either. Thank's for the help, but, it's probably off to the shop and have a real mechanic look at it.

Isn't this what has been wrong in the trade for just about as long as there have been cars? Instead of coming up with a repeatable routine to prove what is wrong why don't we just jump to the conclusion of the day? When I teach techs how to discern vibrations understanding the frequency of it is the first most critical stage in the identification. There is a lot of science to auto repair today, in fact there was a lot of science to it back in the seventies when I first became a tech but it was rarely looked at that way. FWIW I won't let an employee or a student jump to a conclusion like this, they must have proof or the ticket doesn't get sold.

You're not teaching Tech's on this web site. Some of these people do not have a clue or mechanical ability to understand what your talking about. Keep it simple; nobody is impressed with answers they don't even understand. Keep the classroom at the classroom. PS; I was working on Jets and Helicopters in the '70's as well.

now now men. no need to get nasty. I appreciate all the input. I learned something from all of your responses. Thank-you all.