Just an update, rode about 200 mi last weekend to and from my " go to " riding location, south east Ohio. On the way there are 60-70 mph four lane roads . Still noticed the high speed vibration coming from the seat . Fairly relaxed pace till I get to my first destination, rt 550 . After that didn't really notice the seat vibrations, but this is where I start picking up pace pretty good. Mine has the factory 1st-2nd gear power limitation , but in 3rd gear and above when you normally get that intense rush of speed at 5000 rpm , the thrust of speed is just not there. The motor seems to be running fine but it's almost like something is holding back the drive of the motor , maybe something binding or not turning freely. On the ride back home on 70+ mph freeway the high speed vibration from the seat was gone , but felt more of a rumble from the rear end , almost like a out of balance rear wheel . Got home , no final drive leaks so that's good , checked wheel weights , all are in place . Put up on center stand and wheel seems to be true & straight and turning freely with no binding. I've had this bike for five years and I'm so used to everything being normally very smooth. Engine , transmission , driveshaft , everything works together so smooth and seamlessly with this bike and freeway riding is very comfortable, great high speed touring bike . Something is definitely not the same as it was before the driveshaft replacement . Maybe replacement driveshaft has a larger rotating mass that isn't balanced properly , a installation pin or spline, something not lined up or not installed properly. Am going to check a couple other things this week on rear , after that will have to contact dealer . May send a email to Honda USA first .