One, I've heard that rotation the driveshaft can make a difference. (Remove the bolts and rotate the drive shaft a 1/4 turn, then bolt back in and try)
If that don't work...
Get a angle gage, you should be able to find one at the local hardware. Pull the drive shaft. Now measure the the angle of the differential flange and the angle of the out put shaft. They should be withing 6 degrees of eachother or they will cause a vibration.
Note: There are those on the board that have gone way beond 6 degrees and claim not to have had a problem. Myself I had a horible vibration on deceleration until I got the angles within 6 degrees and havent had a problem sence.