Vibration,,thinking out loud (update)

Jack up the vehicle and remove the rear wheels. Put the lug nuts back on if needed. Start and test through 5th gear. If it still vibrates you have eliminated the rear wheels and tires. Next, remove the drive shaft and test again. If it still vibrates then you have eliminated the driveshaft ahd differential. Out comes the transmission. Be sure to check the clutch pilot bearing while you have the transmission out.

Dave.