I have run out of ideas and need help. Replaced the stock gears with 3.73 (Ford Motosports) in my 1999 Cobra (46K miles) and immediately noticed a bad vibration above 70 mph (through the seat). Re-indexed the driveshaft and this reduced vibration, but still noticable. Replaced the driveshaft with aluminum unit and no difference. Switched to a new set of summer wheels and tires (Enkei/Falken 615) and vibration was reduced fair amount. Even starnger, the vibration moved from 70 to 80 mph. Had shop put car on lift, ran speedo up to 80 mph and inspected driveline- differential felt smooth, tires and driveshaft looked to be true, but could feel the vibration on car body. Bearings felt solid with no play in any of the hubs. Another clue- vibration more noticable when I take foot off of accelerator. Before gear change I had car to +120 mph on race track with no vibration. Any ideas?