I am looking for some help on how to isolate the problem. At or above 75mph in 5th gear ONLY, I get a repeating, cyclical driveline vibration. If I push the clutch in or put the car in neutral, the vibration goes away. I also noted the rpms the vibration occurred at in 5th gear and the vibration does not happen at the noted rpms in 4th, 3rd, etc. I do have energy suspension poly motor and trans mounts. The trans mount has several washers between it and the cross member to raise the rear of my tremec 3550 to get the pinion angle within spec. I had ASAP measure my pinion angles since my car is lowereed 1 inch and they are within spec. I have a brand new FRPP aluminum driveshaft and a metal extension to compensate for the shorter 3550. I just had the tires balanced with no help. I did notice the right side O2 sensor is almost touching my FRPP safety bellhouse, but I don't think that is the problem or the vibration would be happening in all gears. Any advice on how to determine if the problem is in the 3550, rear axle or driveshaft would be appreciated.