[fredjacksonsan]

One other test I can think of: when the vibration starts at highway speed, I'm thinking you're in top gear. Try downshifting to the next gear and maintain speed. If it's the driveshaft, you'll keep the vibration and if the TC, then it will likely go away. Play with it a bit to vary the load on the drivetrain, if it goes away try to recreate it in the lower gear.