T800 rear end vibration

Just my two cents, but I had a similar problem and it was the slip yoke on my driveshaft. Slide underneath and put some force on the shaft. There should be no movement at all in a slip yoke except in and out.

 

The first thing I would recommend is contact your Kenworth dealer with the VIN and get them to print off all the driveshaft and drive train component angles. Ask them for the dealer/mechanics "drive line angle worksheet". Then see how close they actually are. The working angles of the u-joints should be the same on both ends of each driveshaft. When a driveshaft rotates the U-joint angle causes the shaft to change speed slightly, speeding up and slowing down with every 1/4 of a rotation. If the U-joint working angles on either end of the drive shaft are not the same then both ends of the driveshaft are trying to turn at different speeds which causes a vibration. There is a bit of leeway on the angles if I remember correctly plus or minus 1/2 a degree. The U-joint at the rear of the transmission does not have to be zero degrees. So if the engine transmission is installed at 2 degrees and the front driveshaft is installed at 1 degree then the working angle of the U-joint at the rear of the transmission is 1 degree, Then the U-joint behind the center bearing and the U-joint in front of the front diff should both have a working angle of 1 degree. Again plus or minus 1/2 a degree. Also the inter-axle driveshaft angles need to be checked. Measure from the machined surface where the diff carrier housing meets the axle housing on both diffs and measure the angle of the inter-axle shaft the working angles on both ends should be the same.
You can make a spacer to lower the center bearing raising it is a bit more work drilling the bracket to raise it.You can use 5/8 hard washers for shims on the top torque rods on the axles to adjust the axle angles.

Have you had the driveshafts checked for twist? A very slight twist in a driveshaft can also cause a vibration.
What style of yokes do you have?