Vibration at or around 60 mph is usually rear wheels out of balance. You'll feel it through the seat rather than through the steering (which would be front wheels) and you'll probably find you can drive through it and it'll stop at about 70. Torque converter wouldn't be road speed related but engine speed related.
+1 on U joints.
These things really are hard on them, and no one greases them as much as necessary.....Myself included! The over 60 vibration was just what I had. I checked them, and found the front to be completely totaled, as in a few more miles, and I would be looking at a new tranny and or transfer case when it disintegrated and became a flailing pipe!.
RR Toadie says "Grease them whenever you change the oil"...........(You have to change the oil? 8-0=)