Just a quick bit of info. In my experience with drive shafts and u-joints vibration under load usually means a bad u-joint. In all my years of driving(30+) I have never had this issue without the associated loose/sloppy feel in the u-joint when you grab and try to move it around...until the last time. I had the same issue you're talking about, vibration under load. I crawled under the truck, grabbed the u-joint and tried to move/shake it around. But no dice, tight as a drum. Now I was thinking oh great, I must have messed up my drive shaft. Then I decided that u-joints are cheaper than a new drive shaft so I thought I would try that first. I picked up new joints and when I pulled the drive shaft I was surprised to find that the rear joint was good in 3 of the cup/bearings, but 1 of them was dry and had basically no needle bearings left. I replaced both joints and no more vibration! So to sum up this long rambling story, don't discount the u-joints just because they may not "feel" bad.