Sorry to Hi-jack a thread, but instead of me making one, I figured I'd post here since I'm basically having the EXACT same problem. Mine occurs at slightly different speeds.
I've got a 2001 3.4L Doublecab w/ auto tranny w/ 141k miles. I fee absolutely NO vibration until I reach roughly 60-65 MPH. After that, I seem to get more of a pulsation vibration than just a vibration. One thing I will mention is that I didn't have this vibration before my recent frame replacement, but I'm not sure that has anything to do with it.
I've checked all the U-joints (tried to shake the hell out of them) but none of them seem to move at all. I checked all of the oil in the diff's and transfer case just to be sure. Everything seemed good there. Also jacked up the front end, tire by tire to check wheel bearings and didn't feel any play there. The front CV joints are new ones from Oreilly (originals had torn cv boots).
Now I did replace the center bearing (carrier bearing) with one from Oreilly Auto HB-26, but after reading the carrier replacement thread, there are a few things that I didn't do that maybe I need too. I was in a hurry so I didn't make the alignment of where the shafts came off at. I know that the shaft to the rear end and the shaft to the transfer case are either right on or 180 out because of how the dirt was. but at the same time, I do know for a fact that the carrier bearing section is 90* off in one direction or the other.
The other part that I noticed I didn't do was torque part on the nut that holds the carrier bearing on. I didn't torque mine, then back it off and retorque it. I just torqued it up and left it. Think it could be to much torque? I will pull the whole thing off, and try and get it lined up and torque it right this time.
But, even with the things that I did wrong on the carrier replacement, my vibration is still in the same spot and acting like before. Around 65mph it's just a pulsation. Almost the same to that if you are driving on the drunk bumps on the edge of the road. You get that WOMP WOMP WOMP WOMP WOMP sound. One that that I did notice was that it seem more pronounced that if you are going around a left hand curve at 65-70mph. Seems like it's worse. Almost makes me wanna think that something is off in the front end. I didn't feel anything and when the frame was replaced and the front end alignment done (which i'm assuming they done) they didn't say anything about tie rods or anything being bad.
I'm going to play with it some more tomorrow, but If i can't find anything, it's headed to the shop on Monday. Gotta get it fixed before the drive to North Dakota.
So other than the carrier bearing, what would everyone else check that I haven't listed in this post.
Again, sorry for the Hi-jack.