HELP!!!!!!! STILL HAVE VIBRATION

Okay, brought the drive shaft to a driveline shop and they don't see anything wrong. They said the hanger bearing is like new and the diveshaft is not out of balance. Four new tires, three new u-joints, new torque converter. Now remember from previous posts that you can fill it from about 35-50mph witjout a load and with my utility trailer from about 20-55 mph. What else can it be? I am on the verge of trading it in just to get rid of the problem. Please any help would be appreciated. Thanks for the help in advance.