I replaced both u-joints

Did you mark the yokes so they could be assembled the same way? If not, first try unbolting the rear u-joint, then spin the drive shaft half a turn and reassemble it. Next, check if an ear got bent on the rear when a bearing cup was pressed in or out. Check if a weight got knocked off the shaft. You'll see a shiny spot at one end. Don't overlook a needle bearing that fell over when the cup was being installed. That has happened to me many times but usually you won't be able to press the cup all the way in, and if you force it, the back of the cup will blow out. Were you able to spin both joints freely after they were installed? If one was tight, you have to tap on the metal loops to shock the bearing cups a little so they free up.