I just replaced my driveshaft about months ago. The sound / vibration only occured between 10-20 mph, and then it would stop. I don't remember if It felt odd during shifts but the vibration increased with the gas (10-20). I suspected that it was the pilot bearing or center bearing whatever you want to call it, I put the car up on stands and inspected the shaft after removing the heat sheild and did not notice anything out of place. I then pulled the driveshaft and inspected it and that is when I saw the fouled u-joint, it would only swivel in one direction and was completely frozen in the center otherwise (output shaft end). One thing that I did that kind of confirmed my suspicion was to put the fwd fuse in, Immediately afterward the vibration/sound went away, I could still feel the driveshaft wobble just from being pulled along( before I pulled the shaft of course). so I would suggest trying the fuse thing and see what happens if you have awd that is. Sorry to be so long winded, I hope this helps you out some good luck and post back with any questions that you may have.