Vibration issues

For my new driveshaft they reused one of the ends of my old driveshaft. The ends still fit the u joints without any play. They didn't use one of the ends because on my old driveshaft the driveshaft tube had two different diameters (it had a step down in the tube from 3-1/4" on one end to 3" on the other) and the new driveshaft is a constant 3" diameter.
Byron-
-I am with cudafacthackjob on getting the driveline high speed balanced with the u-joints and slip yoke attached- especially if it hasn't been done since the driveline was thrown off.
-The old and new slip yokes I had both were smooth inside before the spline grooves started. Think that is standard. I do know that the new slip yoke was an inch or so shorter in the overall shaft length. I read somewhere that Chrysler shortened the design of the Slip yoke from 8+" to 7+" in order to address a vibration concern but that is a third party unconfirmed report. It may help explain why the new slip yokes come shorter than they did (at least on my 68 barracuda with a 727 trans.
-I recommend Drivelines NW after having them do my Driveline. They have three locations in the area and I think there is one in Everett that would be closest to you.